A work report of the code written for mlpack under Google Summer of Code 2018
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README.md

README.md

Organisation: mlpack

Project: Variational Autoencoders

Mentor: Sumedh Ghaisas

mlpack is a C++ machine learning library with emphasis on scalability, speed, and ease-of-use. Its aim is to make machine learning possible for novice users by means of a simple, consistent API, while simultaneously exploiting C++ language features to provide maximum performance and maximum flexibility for expert users. This is done by providing a set of command-line executables which can be used as black boxes, and a modular C++ API for expert users and researchers to easily make changes to the internals of the algorithms. In addition to its powerful C++ interface, mlpack also provides Python bindings.

Objectives:

The objective of this project was to add complete support for Variational Autoencoders(VAEs) and all its derivatives to the library, construct models and reproduce results given in particular research papers.

During the proposal period, I had planned to implement an independent class for VAEs. However, Sumedh suggested that to make it truly generic and to allow all sorts of hierarchical models, it will be better if we implemented the extra components needed and allow them to integrate seamlessly with the rest of the codebase. We went ahead with that plan and now mlpack can be used to implement almost all VAE architectures.

Support for conditional VAEs needs a new layer for which I have opened a PR. I had also planned to test some recurrent VAE models which I will continue to do after GSoC.

Overview:

The project can now be broken down into 3 major tasks:

  1. Implemented a Reparametrization(Sampling) layer which includes Kullback Leibler loss. A visitor was added to collect extra losses(in this case KL) from all the layers. It supports beta-VAEs.
  2. Implemented a ReconstructionLoss class. Along with it, two model distributions were implemented. Bernoulli distribution has been merged. Normal distribution, though tested rigorously, couldn't model the MNIST dataset well, so the PR has been kept on hold.
  3. Implemented multilayer perceptron and convolutional VAE models(in the mlpack/models repository) and trained them on MNIST and Binary MNIST. Wrote some generation scripts and experimented with the learned distribution. Tested beta-VAEs. While training it on a much bigger CelebA dataset, ran into performance issues. Opened a PR improving the TransposedConvolution layer. For conditional VAEs, implemented a new Concatenate layer.

Weekly updates I made on mlpack's blog.

Code:

The pull requests opened in the GSoC period that have been merged :

Extend support of Softplus function to matrices, cubes

Add a Sampling layer, implement KL divergence in it

Remove redundant data members, move NegativeLogLikelihood to loss folder

Add ReconstructionLoss

Make changes suggested in sampling PR

Overload Forward() function

Add Bernoulli distribution and support for beta-VAEs

Commits(Includes commits to the library before GSoC started)

The pull requests opened in the GSoC period that are currently being reviewed:

Add NormalDistribution class

[WIP] Add Variational Autoencoder model for MNIST (in the mlpack/models repository)

Improve TransposedConvolution layer

[WIP] Add Concatenate layer.

Results:

Here are some results that have been generated after training on the MNIST dataset:

The above samples are the result of modifying each of the 10 latent variables independently.

The above samples are the result of varying 2 latent variables in 2D.

Sampling from the model trained on Binary MNIST:

The above samples are from the prior.

The above samples are from the posterior.

Conclusion:

I can't believe how much I learned in the last 3 months. It has been an awesome experience working on this project. There isn't a better way to learn machine learning algorithms than to implement them from the ground up. Huge thanks to Sumedh for the help and guidance, I got to learn a lot from him. He always gave enough time to clear my doubts and helped me when I was stuck. Also, he gave a better direction to the project than what I was planning to go for. I also thank Marcus(github) and Ryan(github) for all the prompt help and always clearing whatever doubts I had. This project would have been a lot tougher and longer without you all to guide me.

I strongly intend to continue contributing to mlpack in all the ways I can, because it's just too much fun.

Also, thanks to Google for this amazing opportunity and the generous funding.