PyTorch continues to gain momentum because of its focus on meeting the needs of researchers, its streamlined workflow for production use, and most of all because of the enthusiastic support it has received from the AI community. PyTorch citations in papers on ArXiv grew 194 percent in the first half of 2019 alone, as noted by O’Reilly, and the number of contributors to the platform has grown more than 50 percent over the last year, to nearly 1,200. Facebook, Microsoft, Uber, and other organizations across industries are increasingly using it as the foundation for their most important machine learning (ML) research and production workloads.

We are now advancing the platform further with the release of PyTorch 1.3, which includes experimental support for features such as seamless model deployment to mobile devices, model quantization for better performance at inference time, and front-end improvements, like the ability to name tensors and create clearer code with less need for inline comments. We’re also launching a number of additional tools and libraries to support model interpretability and bringing multimodal research to production.

Additionally, we’ve collaborated with Google and Salesforce to add broad support for Cloud Tensor Processing Units, providing a significantly accelerated option for training large-scale deep neural networks. Alibaba Cloud also joins Amazon Web Services, Microsoft Azure, and Google Cloud as supported cloud platforms for PyTorch users. You can get started now at pytorch.org.

PyTorch 1.3

The 1.3 release of PyTorch brings significant new features, including experimental support for mobile device deployment, eager mode quantization at 8-bit integer, and the ability to name tensors. With each of these enhancements, we look forward to additional contributions and improvements from the PyTorch community.

Named tensors (experimental)

Cornell University’s Sasha Rush has argued that, despite its ubiquity in deep learning, the traditional implementation of tensors has significant shortcomings, such as exposing private dimensions, broadcasting based on absolute position, and keeping type information in documentation. He proposed named tensors as an alternative approach.

Today, we name and access dimensions by comment:

# Tensor[N, C, H, W]
 images = torch.randn(32, 3, 56, 56)
 images.sum(dim=1)
 images.select(dim=1, index=0)

But naming explicitly leads to more readable and maintainable code:

NCHW = [‘N’, ‘C’, ‘H’, ‘W’]
   images = torch.randn(32, 3, 56, 56, names=NCHW)
   images.sum('C')
   images.select('C', index=0)

Quantization (experimental)

It’s important to make efficient use of both server-side and on-device compute resources when developing ML applications. To support more efficient deployment on servers and edge devices, PyTorch 1.3 now supports 8-bit model quantization using the familiar eager mode Python API. Quantization refers to techniques used to perform computation and storage at reduced precision, such as 8-bit integer. This currently experimental feature includes support for post-training quantization, dynamic quantization, and quantization-aware training. It leverages the FBGEMM and QNNPACK state-of-the-art quantized kernel back ends, for x86 and ARM CPUs, respectively, which are integrated with PyTorch and now share a common API.

To learn more about the design and architecture, check out the API docs here, and get started with any of the supported techniques using the tutorials available here.

PyTorch mobile (experimental)

Running ML on edge devices is growing in importance as applications continue to demand lower latency. It is also a foundational element for privacy-preserving techniques such as federated learning. To enable more efficient on-device ML, PyTorch 1.3 now supports an end-to-end workflow from Python to deployment on iOS and Android.

This is an early, experimental release, optimized for end-to-end development. Coming releases will focus on:

• Optimization for size: Build level optimization and selective compilation depending on the operators needed for user applications (i.e., you pay binary size for only the operators you need)
• Performance: Further improvements to performance and coverage on mobile CPUs and GPUs
• High level API: Extend mobile native APIs to cover common preprocessing and integration tasks needed for incorporating ML in mobile applications. e.g. Computer vision and NLP

Learn more or get started on Android or iOS here.

New tools for model interpretability and privacy

Captum

As models become ever more complex, it is increasingly important to develop new methods for model interpretability. To help address this need, we’re launching Captum, a tool to help developers working in PyTorch understand why their model generates a specific output. Captum provides state-of-the-art tools to understand how the importance of specific neurons and layers and affect predictions made by the models. Captum’s algorithms include integrated gradients, conductance, SmoothGrad and VarGrad, and DeepLift.

The example below shows how to apply model interpretability algorithms on a pretrained ResNet model and then visualize the attributions for each pixel by overlaying them on the image.

noise_tunnel = NoiseTunnel(integrated_gradients)

attributions_ig_nt, delta = noise_tunnel.attribute(input, n_samples=10, nt_type='smoothgrad_sq', target=pred_label_idx)
_ = viz.visualize_image_attr_multiple(["original_image", "heat_map"],
                                      ["all", "positive"],
                                      np.transpose(attributions_ig_nt.squeeze().cpu().detach().numpy(), (1,2,0)),
                                      np.transpose(transformed_img.squeeze().cpu().detach().numpy(), (1,2,0)),
                                      cmap=default_cmap,
                                      show_colorbar=True)

Learn more about Captum at captum.ai.

CrypTen

Practical applications of ML via cloud-based or machine-learning-as-a-service (MLaaS) platforms pose a range of security and privacy challenges. In particular, users of these platforms may not want or be able to share unencrypted data, which prevents them from taking full advantage of ML tools. To address these challenges, the ML community is exploring a number of technical approaches, at various levels of maturity. These include homomorphic encryption, secure multiparty computation, trusted execution environments, on-device computation, and differential privacy.

To provide a better understanding of how some of these technologies can be applied, we are releasing CrypTen, a new community-based research platform for taking the field of privacy-preserving ML forward. Learn more about CrypTen here. It is available on GitHub here.

Tools for multimodal AI systems

Digital content is often made up of several modalities, such as text, images, audio, and video. For example, a single public post might contain an image, body text, a title, a video, and a landing page. Even one particular component may have more than one modality, such as a video that contains both visual and audio signals, or a landing page that is composed of images, text, and HTML sources.

The ecosystem of tools and libraries that work with PyTorch offer enhanced ways to address the challenges of building multimodal ML systems. Here are some of the latest libraries launching today:

Detectron2

Object detection and segmentation are used for tasks ranging from autonomous vehicles to content understanding for platform integrity. To advance this work, Facebook AI Research (FAIR) is releasing Detectron2, an object detection library now implemented in PyTorch. Detectron2 provides support for the latest models and tasks, increased flexibility to aid computer vision research, and improvements in maintainability and scalability to support production use cases.

Detectron2 is available here and you can learn more here.

Speech extensions to fairseq

Language translation and audio processing are critical components in systems and applications such as search, translation, speech, and assistants. There has been tremendous progress in these fields recently thanks to the development of new architectures like transformers, as well as large-scale pretraining methods. We’ve extended fairseq, a framework for sequence-to-sequence applications such as language translation, to include support for end-to-end learning for speech and audio recognition tasks.These extensions to fairseq enable faster exploration and prototyping of new speech research ideas while offering a clear path to production.

Get started with fairseq here.

Cloud provider and hardware ecosystem support

Cloud providers such as Amazon Web Services, Microsoft Azure, and Google Cloud provide extensive support for anyone looking to develop ML on PyTorch and deploy in production. We’re excited to share the general availability of Google Cloud TPU support and a newly launched integration with Alibaba Cloud. We’re also expanding hardware ecosystem support.

• Google Cloud TPU support now broadly available. To accelerate the largest-scale machine learning (ML) applications deployed today and enable rapid development of the ML applications of tomorrow, Google created custom silicon chips called Tensor Processing Units (TPUs). When assembled into multi-rack ML supercomputers called Cloud TPU Pods, these TPUs can complete ML workloads in minutes or hours that previously took days or weeks on other systems. Engineers from Facebook, Google, and Salesforce worked together to enable and pilot Cloud TPU support in PyTorch, including experimental support for Cloud TPU Pods. PyTorch support for Cloud TPUs is also available in Colab. Learn more about how to get started with PyTorch on Cloud TPUs here.
• Alibaba adds support for PyTorch in Alibaba Cloud. The initial integration involves a one-click solution for PyTorch 1.x, Data Science Workshop notebook service, distributed training with Gloo/NCCL, as well as seamless integration with Alibaba IaaS such as OSS, ODPS, and NAS. Together with the toolchain provided by Alibaba, we look forward to significantly reducing the overhead necessary for adoption, as well as helping Alibaba Cloud’s global customer base leverage PyTorch to develop new AI applications.
• ML hardware ecosystem expands. In addition to key GPU and CPU partners, the PyTorch ecosystem has also enabled support for dedicated ML accelerators. Updates from Intel and Habana showcase how PyTorch, connected to the Glow optimizing compiler, enables developers to utilize these market-specific solutions.

Growth in the PyTorch community

As an open source, community-driven project, PyTorch benefits from wide range of contributors bringing new capabilities to the ecosystem. Here are some recent examples:

• Mila SpeechBrain aims to provide an open source, all-in-one speech toolkit based on PyTorch. The goal is to develop a single, flexible, user-friendly toolkit that can be used to easily develop state-of-the-art systems for speech recognition (both end to end and HMM-DNN), speaker recognition, speech separation, multi-microphone signal processing (e.g., beamforming), self-supervised learning, and many others. Learn more
• SpaCy is a new wrapping library with consistent and easy-to-use interfaces to several models, in order to extract features to power NLP pipelines. Support is provided for via spaCy’s standard training API. The library also calculates an alignment so the transformer features can be related back to actual words instead of just wordpieces. Learn more
• HuggingFace PyTorch-Transformers (formerly known as pytorch-pretrained-bert is a library of state-of-the-art pretrained models for Natural Language Processing (NLP). The library currently contains PyTorch implementations, pretrained model weights, usage scripts, and conversion utilities for models such as BERT, GPT-2, RoBERTa, and DistilBERT. It has also grown quickly, with more than 13,000 GitHub stars and a broad set of users. Learn more
• PyTorch Lightning is a Keras-like ML library for PyTorch. It leaves core training and validation logic to you and automates the rest. Reproducibility is a crucial requirement for many fields of research, including those based on ML techniques. As the number of research papers submitted to arXiv and conferences skyrockets into the tens of thousands, scaling reproducibility becomes difficult. Learn more.

We recently held the first online Global PyTorch Summer Hackathon, where researchers and developers around the world were invited to build innovative new projects with PyTorch. Nearly 1,500 developers participated, submitting projects ranging from livestock disease detection to AI-powered financial assistants. The winning projects were:

• Torchmeta, which provides extensions for PyTorch to simplify the development of meta-learning algorithms in PyTorch. It features a unified interface inspired by TorchVision for both few-shot classification and regression problems, to allow easy benchmarking on multiple data sets to aid with reproducibility.
• Open-Unmix, a system for end-to-end music demixing with PyTorch. Demixing separates the individual instruments or vocal track from any stereo recording.
• Endless AI-Generated Tees, a store featuring AI-generated T-shirt designs that can be purchased and delivered worldwide. The system uses a state-of-the-art generative model (StyleGAN) that was built with PyTorch and then trained on modern art.

Visit pytorch.org to learn more and get started with PyTorch 1.3 and the latest libraries and ecosystem projects. We look forward to the contributions, exciting research advancements, and real-world applications that the community builds with PyTorch.

We’d like to thank the entire PyTorch team and the community for all their contributions to this work.

Read More

Since the release of PyTorch 1.0, we’ve seen the community expand to add new tools, contribute to a growing set of models available in the PyTorch Hub, and continually increase usage in both research and production.

From a core perspective, PyTorch has continued to add features to support both research and production usage, including the ability to bridge these two worlds via TorchScript. Today, we are excited to announce that we have four new releases including PyTorch 1.2, torchvision 0.4, torchaudio 0.3, and torchtext 0.4. You can get started now with any of these releases at pytorch.org.

PyTorch 1.2

With PyTorch 1.2, the open source ML framework takes a major step forward for production usage with the addition of an improved and more polished TorchScript environment. These improvements make it even easier to ship production models, expand support for exporting ONNX formatted models, and enhance module level support for Transformers. In addition to these new features, TensorBoard is now no longer experimental – you can simply type from torch.utils.tensorboard import SummaryWriter to get started.

TorchScript Improvements

Since its release in PyTorch 1.0, TorchScript has provided a path to production for eager PyTorch models. The TorchScript compiler converts PyTorch models to a statically typed graph representation, opening up opportunities for
optimization and execution in constrained environments where Python is not available. You can incrementally convert your model to TorchScript, mixing compiled code seamlessly with Python.

PyTorch 1.2 significantly expands TorchScript’s support for the subset of Python used in PyTorch models and delivers a new, easier-to-use API for compiling your models to TorchScript. See the migration guide for details. Below is an example usage of the new API:

import torch

class MyModule(torch.nn.Module):
    def __init__(self, N, M):
        super(MyModule, self).__init__()
        self.weight = torch.nn.Parameter(torch.rand(N, M))

    def forward(self, input):
        if input.sum() > 0:
          output = self.weight.mv(input)
        else:
          output = self.weight + input
        return output

# Compile the model code to a static representation
my_script_module = torch.jit.script(MyModule(3, 4))

# Save the compiled code and model data so it can be loaded elsewhere
my_script_module.save("my_script_module.pt")

To learn more, see our Introduction to TorchScript and Loading a
PyTorch Model in C++ tutorials.

Expanded ONNX Export

The ONNX community continues to grow with an open governance structure and additional steering committee members, special interest groups (SIGs), and working groups (WGs). In collaboration with Microsoft, we’ve added full support to export ONNX Opset versions 7(v1.2), 8(v1.3), 9(v1.4) and 10 (v1.5). We’ve have also enhanced the constant folding pass to support Opset 10, the latest available version of ONNX. ScriptModule has also been improved including support for multiple outputs, tensor factories, and tuples as inputs and outputs. Additionally, users are now able to register their own symbolic to export custom ops, and specify the dynamic dimensions of inputs during export. Here is a summary of the all of the major improvements:

• Support for multiple Opsets including the ability to export dropout, slice, flip, and interpolate in Opset 10.
• Improvements to ScriptModule including support for multiple outputs, tensor factories, and tuples as inputs and outputs.
• More than a dozen additional PyTorch operators supported including the ability to export a custom operator.
• Many big fixes and test infra improvements.

You can try out the latest tutorial here, contributed by @lara-hdr at Microsoft. A big thank you to the entire Microsoft team for all of their hard work to make this release happen!

nn.Transformer

In PyTorch 1.2, we now include a standard nn.Transformer module, based on the paper “Attention is All You Need”. The nn.Transformer module relies entirely on an attention mechanism to draw global dependencies between input and output. The individual components of the nn.Transformer module are designed so they can be adopted independently. For example, the nn.TransformerEncoder can be used by itself, without the larger nn.Transformer. The new APIs include:

• nn.Transformer
• nn.TransformerEncoder and nn.TransformerEncoderLayer
• nn.TransformerDecoder and nn.TransformerDecoderLayer

See the Transformer Layers documentation for more information. See here for the full PyTorch 1.2 release notes.

Domain API Library Updates

PyTorch domain libraries like torchvision, torchtext, and torchaudio provide convenient access to common datasets, models, and transforms that can be used to quickly create a state-of-the-art baseline. Moreover, they also provide common abstractions to reduce boilerplate code that users might have to otherwise repeatedly write. Since research domains have distinct requirements, an ecosystem of specialized libraries called domain APIs (DAPI) has emerged around PyTorch to simplify the development of new and existing algorithms in a number of fields. We’re excited to release three updated DAPI libraries for text, audio, and vision that compliment the PyTorch 1.2 core release.

Torchaudio 0.3 with Kaldi Compatibility, New Transforms

Torchaudio specializes in machine understanding of audio waveforms. It is an ML library that provides relevant signal processing functionality (but is not a general signal processing library). It leverages PyTorch’s GPU support to provide many tools and transformations for waveforms to make data loading and standardization easier and more readable. For example, it offers data loaders for waveforms using sox, and transformations such as spectrograms, resampling, and mu-law encoding and decoding.

We are happy to announce the availability of torchaudio 0.3.0, with a focus on standardization and complex numbers, a transformation (resample) and two new functionals (phase_vocoder, ISTFT), Kaldi compatibility, and a new tutorial. Torchaudio was redesigned to be an extension of PyTorch and a part of the domain APIs (DAPI) ecosystem.

Standardization

Significant effort in solving machine learning problems goes into data preparation. In this new release, we’ve updated torchaudio’s interfaces for its transformations to standardize around the following vocabulary and conventions.

Tensors are assumed to have channel as the first dimension and time as the last dimension (when applicable). This makes it consistent with PyTorch’s dimensions. For size names, the prefix n_ is used (e.g. “a tensor of size (n_freq, n_mel)”) whereas dimension names do not have this prefix (e.g. “a tensor of dimension (channel, time)”). The input of all transforms and functions now assumes channel first. This is done to be consistent with PyTorch, which has channel followed by the number of samples. The channel parameter of all transforms and functions is now deprecated.

The output of STFT is (channel, frequency, time, 2), meaning for each channel, the columns are the Fourier transform of a certain window, so as we travel horizontally we can see each column (the Fourier transformed waveform) change over time. This matches the output of librosa so we no longer need to transpose in our test comparisons with Spectrogram, MelScale, MelSpectrogram, and MFCC. Moreover, because of these new conventions, we deprecated LC2CL and BLC2CBL which were used to transfer from one shape of signal to another.

As part of this release, we’re also introducing support for complex numbers via tensors of dimension (…, 2), and providing magphase to convert such a tensor into its magnitude and phase, and similarly complex_norm and angle.

The details of the standardization are provided in the README.

Functionals, Transformations, and Kaldi Compatibility

Prior to the standardization, we separated state and computation into torchaudio.transforms and torchaudio.functional.

As part of the transforms, we’re adding a new transformation in 0.3.0: Resample. Resample can upsample or downsample a waveform to a different frequency.

As part of the functionals, we’re introducing: phase_vocoder, a phase vocoder to change the speed of a waveform without changing its pitch, and ISTFT, the inverse STFT implemented to be compatible with STFT provided by PyTorch. This separation allows us to make functionals weak scriptable and to utilize JIT in 0.3.0. We thus have JIT and CUDA support for the following transformations: Spectrogram, AmplitudeToDB (previously named SpectrogramToDB), MelScale,
MelSpectrogram, MFCC, MuLawEncoding, MuLawDecoding (previously named MuLawExpanding).

We now also provide a compatibility interface with Kaldi to ease onboarding and reduce a user’s code dependency on Kaldi. We now have an interface for spectrogram, fbank, and resample_waveform.

New Tutorial

To showcase the new conventions and transformations, we have a new tutorial demonstrating how to preprocess waveforms using torchaudio. This tutorial walks through an example of loading a waveform and applying some of the available transformations to it.

We are excited to see an active community around torchaudio and eager to further grow and support it. We encourage you to go ahead and experiment for yourself with this tutorial and the two datasets that are available: VCTK and YESNO! They have an interface to download the datasets and preprocess them in a convenient format. You can find the details in the release notes here.

Torchtext 0.4 with supervised learning datasets

A key focus area of torchtext is to provide the fundamental elements to help accelerate NLP research. This includes easy access to commonly used datasets and basic preprocessing pipelines for working on raw text based data. The torchtext 0.4.0 release includes several popular supervised learning baselines with “one-command” data loading. A tutorial is included to show how to use the new datasets for text classification analysis. We also added and improved on a few functions such as get_tokenizer and build_vocab_from_iterator to make it easier to implement future datasets. Additional examples can be found here.

Text classification is an important task in Natural Language Processing with many applications, such as sentiment analysis. The new release includes several popular text classification datasets for supervised learning including:

• AG_NEWS
• SogouNews
• DBpedia
• YelpReviewPolarity
• YelpReviewFull
• YahooAnswers
• AmazonReviewPolarity
• AmazonReviewFull

Each dataset comes with two parts (train vs. test), and can be easily loaded with a single command. The datasets also support an ngrams feature to capture the partial information about the local word order. Take a look at the tutorial here to learn more about how to use the new datasets for supervised problems such as text classification analysis.

from torchtext.datasets.text_classification import DATASETS
train_dataset, test_dataset = DATASETS['AG_NEWS'](ngrams=2)

In addition to the domain library, PyTorch provides many tools to make data loading easy. Users now can load and preprocess the text classification datasets with some well supported tools, like torch.utils.data.DataLoader and torch.utils.data.IterableDataset. Here are a few lines to wrap the data with DataLoader. More examples can be found here.

from torch.utils.data import DataLoader
data = DataLoader(train_dataset, collate_fn=generate_batch)

Check out the release notes here to learn more and try out the tutorial here.

Torchvision 0.4 with Support for Video

Video is now a first-class citizen in torchvision, with support for data loading, datasets, pre-trained models, and transforms. The 0.4 release of torchvision includes:

• Efficient IO primitives for reading/writing video files (including audio), with support for arbitrary encodings and formats.
• Standard video datasets, compatible with torch.utils.data.Dataset and torch.utils.data.DataLoader.
• Pre-trained models built on the Kinetics-400 dataset for action classification on videos (including the training scripts).
• Reference training scripts for training your own video models.

We wanted working with video data in PyTorch to be as straightforward as possible, without compromising too much on performance.
As such, we avoid the steps that would require re-encoding the videos beforehand, as it would involve:

• A preprocessing step which duplicates the dataset in order to re-encode it.
• An overhead in time and space because this re-encoding is time-consuming.
• Generally, an external script should be used to perform the re-encoding.

Additionally, we provide APIs such as the utility class, VideoClips, that simplifies the task of enumerating all possible clips of fixed size in a list of video files by creating an index of all clips in a set of videos. It also allows you to specify a fixed frame-rate for the videos. An example of the API is provided below:

from torchvision.datasets.video_utils import VideoClips

class MyVideoDataset(object):
    def __init__(self, video_paths):
        self.video_clips = VideoClips(video_paths,
                                      clip_length_in_frames=16,
                                      frames_between_clips=1,
                                      frame_rate=15)

    def __getitem__(self, idx):
        video, audio, info, video_idx = self.video_clips.get_clip(idx)
        return video, audio

    def __len__(self):
        return self.video_clips.num_clips()

Most of the user-facing API is in Python, similar to PyTorch, which makes it easily extensible. Plus, the underlying implementation is fast — torchvision decodes as little as possible from the video on-the-fly in order to return a clip from the video.

Check out the torchvision 0.4 release notes here for more details.

We look forward to continuing our collaboration with the community and hearing your feedback as we further improve and expand the PyTorch deep learning platform.

We’d like to thank the entire PyTorch team and the community for all of the contributions to this work!

Read More

With roads in developed countries like the US changing up to 15% annually, Mapillary addresses a growing demand for keeping maps updated by combining images from any camera into a 3D visualization of the world. Mapillary’s independent and collaborative approach enables anyone to collect, share, and use street-level images for improving maps, developing cities, and advancing the automotive industry.

Today, people and organizations all over the world have contributed more than 600 million images toward Mapillary’s mission of helping people understand the world’s places through images and making this data available, with clients and partners including the World Bank, HERE, and Toyota Research Institute.

Mapillary’s computer vision technology brings intelligence to maps in an unprecedented way, increasing our overall understanding of the world. Mapillary runs state-of-the-art semantic image analysis and image-based 3d modeling at scale and on all its images. In this post we discuss two recent works from Mapillary Research and their implementations in PyTorch – Seamless Scene Segmentation [1] and In-Place Activated BatchNorm [2] – generating Panoptic segmentation results and saving up to 50% of GPU memory during training, respectively.

Seamless Scene Segmentation

Github project page: https://github.com/mapillary/seamseg/

The objective of Seamless Scene Segmentation is to predict a “panoptic” segmentation [3] from an image, that is a complete labeling where each pixel is assigned with a class id and, where possible, an instance id. Like many modern CNNs dealing with instance detection and segmentation, we adopt the Mask R-CNN framework [4], using ResNet50 + FPN [5] as a backbone. This architecture works in two stages: first, the “Proposal Head” selects a set of candidate bounding boxes on the image (i.e. the proposals) that could contain an object; then, the “Mask Head” focuses on each proposal, predicting its class and segmentation mask. The output of this process is a “sparse” instance segmentation, covering only the parts of the image that contain countable objects (e.g. cars and pedestrians).

To complete our panoptic approach coined Seamless Scene Segmentation, we add a third stage to Mask R-CNN. Stemming from the same backbone, the “Semantic Head” predicts a dense semantic segmentation over the whole image, also accounting for the uncountable or amorphous classes (e.g. road and sky). The outputs of the Mask and Semantic heads are finally fused using a simple non-maximum suppression algorithm to generate the final panoptic prediction. All details about the actual network architecture, used losses and underlying math can be found at the project website for our CVPR 2019 paper [1].

While several versions of Mask R-CNN are publicly available, including an official implementation written in Caffe2, at Mapillary we decided to build Seamless Scene Segmentation from scratch using PyTorch, in order to have full control and understanding of the whole pipeline. While doing so we encountered a couple of main stumbling blocks, and had to come up with some creative workarounds we are going to describe next.

Dealing with variable-sized tensors

Something that sets aside panoptic segmentation networks from traditional CNNs is the prevalence of variable-sized data. In fact, many of the quantities we are dealing with cannot be easily represented with fixed sized tensors: each image contains a different number of objects, the Proposal head can produce a different number of proposals for each image, and the images themselves can have different sizes. While this is not a problem per-se – one could just process images one at a time – we would still like to exploit batch-level parallelism as much as possible. Furthermore, when performing distributed training with multiple GPUs, DistributedDataParallel expects its inputs to be batched, uniformly-sized tensors.

Our solution to these issues is to wrap each batch of variable-sized tensors in a PackedSequence. PackedSequence is little more than a glorified list class for tensors, tagging its contents as “related”, ensuring that they all share the same type, and providing useful methods like moving all the tensors to a particular device, etc. When performing light-weight operations that wouldn’t be much faster with batch-level parallelism, we simply iterate over the contents of the PackedSequence in a for loop. When performance is crucial, e.g. in the body of the network, we simply concatenate the contents of the PackedSequence, adding zero padding as required (like in RNNs with variable-length inputs), and keeping track of the original dimensions of each tensor.

PackedSequences also help us deal with the second problem highlighted above. We slightly modify DistributedDataParallel to recognize PackedSequence inputs, splitting them in equally sized chunks and distributing their contents across the GPUs.

Asymmetric computational graphs with Distributed Data Parallel

Another, perhaps more subtle, peculiarity of our network is that it can generate asymmetric computational graphs across GPUs. In fact, some of the modules that compose the network are “optional”, in the sense that they are not always computed for all images. As an example, when the Proposal head doesn’t output any proposal, the Mask head is not traversed at all. If we are training on multiple GPUs with DistributedDataParallel, this results in one of the replicas not computing gradients for the Mask head parameters.

Prior to PyTorch 1.1, this resulted in a crash, so we had to develop a workaround. Our simple but effective solution was to compute a “fake forward pass” when no actual forward is required, i.e. something like this:

def fake_forward():
    fake_input = get_correctly_shaped_fake_input()
    fake_output = mask_head(fake_input)
    fake_loss = fake_output.sum() * 0
    return fake_loss

Here, we generate a batch of bogus data, pass it through the Mask head, and return a loss that always back-progates zeros to all parameters.

Starting from PyTorch 1.1 this workaround is no longer required: by setting find_unused_parameters=True in the constructor, DistributedDataParallel is told to identify parameters whose gradients have not been computed by all replicas and correctly handle them. This leads to some substantial simplifications in our code base!

In-place Activated BatchNorm

Github project page: https://github.com/mapillary/inplace_abn/

Most researchers would probably agree that there are always constraints in terms of available GPU resources, regardless if their research lab has access to only a few or multiple thousands of GPUs. In a time where at Mapillary we still worked at rather few and mostly 12GB Titan X – style prosumer GPUs, we were searching for a solution that virtually enhances the usable memory during training, so we would be able to obtain and push state-of-the-art results on dense labeling tasks like semantic segmentation. In-place activated BatchNorm is enabling us to use up to 50% more memory (at little computational overhead) and is therefore deeply integrated in all our current projects (including Seamless Scene Segmentation described above).

When processing a BN-Activation-Convolution sequence in the forward pass, most deep learning frameworks (including PyTorch) need to store two big buffers, i.e. the input x of BN and the input z of Conv. This is necessary because the standard implementations of the backward passes of BN and Conv depend on their inputs to calculate the gradients. Using InPlace-ABN to replace the BN-Activation sequence, we can safely discard x, thus saving up to 50% GPU memory at training time. To achieve this, we rewrite the backward pass of BN in terms of its output y, which is in turn reconstructed from z by inverting the activation function.

The only limitation of InPlace-ABN is that it requires using an invertible activation function, such as leaky relu or elu. Except for this, it can be used as a direct, drop-in replacement for BN+activation modules in any network. Our native CUDA implementation offers minimal computational overhead compared to PyTorch’s standard BN, and is available for anyone to use from here: https://github.com/mapillary/inplace_abn/.

Synchronized BN with asymmetric graphs and unbalanced batches

When training networks with synchronized SGD over multiple GPUs and/or multiple nodes, it’s common practice to compute BatchNorm statistics separately on each device. However, in our experience working with semantic and panoptic segmentation networks, we found that accumulating mean and variance across all workers can bring a substantial boost in accuracy. This is particularly true when dealing with small batches, like in Seamless Scene Segmentation where we train with a single, super-high resolution image per GPU.

InPlace-ABN supports synchronized operation over multiple GPUs and multiple nodes, and, since version 1.1, this can also be achieved in the standard PyTorch library using SyncBatchNorm. Compared to SyncBatchNorm, however, we support some additional functionality which is particularly important for Seamless Scene Segmentation: unbalanced batches and asymmetric graphs.

As mentioned before, Mask R-CNN-like networks naturally give rise to variable-sized tensors. Thus, in InPlace-ABN we calculate synchronized statistics using a variant of the parallel algorithm described here, which properly takes into account the fact that each GPU can hold a different number of samples. PyTorch’s SyncBatchNorm is currently being revised to support this, and the improved functionality will be available in a future release.

Asymmetric graphs (in the sense mentioned above) are another complicating factor one has to deal with when creating a synchronized BatchNorm implementation. Luckily, PyTorch’s distributed group functionality allows us to restrict distributed communication to a subset of workers, easily excluding those that are currently inactive. The only missing piece is that, in order to create a distributed group, each process needs to know the ids of all processes that will participate in the group, and even processes that are not part of the group need to call the new_group() function. In InPlace-ABN we handle it with a function like this:

import torch
import torch.distributed as distributed

def active_group(active):
    """Initialize a distributed group where each process can independently decide whether to participate or not"""
    world_size = distributed.get_world_size()
    rank = distributed.get_rank()

    # Gather active status from all workers
    active = torch.tensor(rank if active else -1, dtype=torch.long, device=torch.cuda.current_device())
    active_workers = torch.empty(world_size, dtype=torch.long, device=torch.cuda.current_device())
    distributed.all_gather(list(active_workers.unbind(0)), active)

    # Create group
    active_workers = [int(i) for i in active_workers.tolist() if i != -1]
    group = distributed.new_group(active_workers)
    return group

First each process, including inactive ones, communicates its status to all others through an all_gather call, then it creates the distributed group with the shared information. In the actual implementation we also include a caching mechanism for groups, since new_group() is usually too expensive to call at each batch.

References

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