Welcome back guys.

Today brings a tutorial on how to make a text variational autoencoder (VAE) in Keras with a twist. Instead of just having a vanilla VAE, we’ll also be making predictions based on the latent space representations of our text. The model will be trained on the IMDB dataset available in Keras, and the goal of the model will be to simultaneously reconstruct movie reviews and predict their sentiment.

Note that due to the large size of the dataset, a very large amount of RAM is required to train the model using many words. I have 64GB and even that isn’t enough since we need one hot encodings of the reviews which results in a shape of `(num_reviews, max_review_len, vocab_size)`

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## Basic VAE Knowledge

In case some of you don’t know how VAEs work, I’m going to briefly describe the idea here. Feel free to skip this section if you’re familiar with the concept.

Before continuing with the description of how VAEs work, let’s first discuss what this so called latent space is. We begin by illustrating the similarity in expressiveness between human languages. The purpose of text is to convey some knowledge or meaning, and this is what is achieved via human languages such as English, Arabic, or even Japanese. Notice how syntacically different these languages are, yet they’re all able to convey meaning and knowledge (as long as the reader knows the language).

The fact that these languages are so different, and that people are able to communicate just as well with either one of them suggests that there is something going on behind the scenes. This tells us that language is a sort of physical realization of abstract concepts like intention, knowledge, and emotion. Thus, what matters when determining whether a movie review is negative or positive depends on the ideas conveyed in that review, and not the particular English words. This brings us to the concept of a latent space. For the model we are using, the latent space contains the meaning of movie reviews encoded as numerical vectors, and it is these vectors that are used to determine if a review is positive as well as reconstructing the original review.

A variational autoencoder is similar to a regular autoencoder except that it is a generative model. This “generative” aspect stems from placing an additional constraint on the loss function such that the latent space is spread out and doesn’t contain dead zones where reconstructing an input from those locations results in garbage. By doing this, we can randomly sample a vector from the latent space and hopefully create a meaninful decoded output from it.

The “variational” part comes from the fact that we’re trying to approximate the posterior distribution \( p_{\theta}(z | x) \) with a variational distribution \( q_{\phi}(z | x) \). Thus, the encoder outputs parameters to this variational distribution which is just a multivariate Gaussian distribution, and the latent representation is obtained by then sampling this distribution. The decoder then takes the latent representation and tries to reconstruct the original input from it.

## Python Code

Here is the source code for the Keras model used to solve the problem mentioned at the beginning of this blog post.

### Model

The model we are using is a consists of 3 distinct components

- A bidirectional RNN encoder
- A simple linear single-layer fully-connected classification network
- An RNN decoder

The choice to have a bidirectional RNN encoder has to do with RNNs being better able to represent the more recent parts of the input sequence in their hidden states. By using a bidirectional RNN where the hidden states are concatenated, we mitigate the issue of not being able to remember the earliest parts of the sequence.

The model.py file is quite large, so we’ll explore it section by section.

#### Imports

Naturally, we have several features from Keras that must be imported due to the complexity of the model.

#### Main Components

Let’s break things down one at a time.

The following simply takes the input sequences and converts them into sequences of learned word embeddings. In a nutshell, these embeddings are vector representations of words, and it can be shown that langauge models tend to have semantically similar words close together in embedding space with PCA and tSNE (not to be confused with latent space).

Then, we create the encoder and VAE loss function. Note `self._build_encoder(x_embed, latent_rep_size=latent_rep_size, max_length=max_length)`

will be defined later.

Now we create the sentiment prediction model based on the encoded latent space representation. Again, `self._build_sentiment_predictor(encoded_input)`

will be defined later.

Based on the encoded latent space representation, we can create the decoder. Again, `self._build_decoder(encoded, vocab_size, max_length)`

will be defined later.

Finally, we build the actual autoencoder itself. Notice how there are 2 outputs, one for the reconstructed input and one for the predicted sentiment.

As promised, we now define the methods that build the encoder, decoder, and sentiment prediction model. Note that these are all part of the VAE class created above.

#### _build_encoder(self, x, latent_rep_size=200, max_length=300, epsilon_std=0.01)

We’ll start off with the encoder since it is the most complicated one. Essentially, the encoder is a stacked bidirectional RNN that then outputs parameters to be used by the `sampling()`

function. The resulting parameters are named `z_mean`

and `z_log_var`

. The sampling function simply takes a random sample of the appropriate size from a multivariate Gaussian distribution. Lastly, the VAE loss is just the standard reconstruction loss (cross entropy loss) with added KL-divergence loss.

#### _build_decoder(self, encoded, vocab_size, max_length)

The decoder is simpler. What it does it take a latent space representation, repeat it for the maximum sequence length, and use that as input to a stacked RNN. The output of the stacked RNN is then fed into a Dense layer which is applied to every single time step of the RNN output using the TimeDistributed wrapper. Notice how the Dense layer has a softmax activation since we will be outputting probabilities for the words in our vocabulary.

If the above decoder looks like its shape doesn’t match the shape of our input, you’ve made an astute observation. Recall that our input will have shape `(batch_size, max_length)`

since that is a requirement for the Embedding layer. However, the output of the decoder has shape `(batch_size, max_length, vocab_size)`

since this is a requirement for the cross entropy part of the VAE loss function. This means that when we train the model, the input will be a numpy array where each row is a training example, and every column is a numerical index bounded by the number of words in our vocabulary. The target output will just be a one-hot representation of the input matrix having shape `(num_examples, max_length, vocab_size)`

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#### _build_sentiment_predictor(self, encoded)

Last but not least, let’s look at the sentiment prediction model. It is as simple as possible so that the latent space should in theory form two clearly separated regions based on sentiment when projected with PCA. If the prediction model has too high a capacity, then the latent space won’t be encouraged to form a linearly separable shape. Notice that the final Dense layer has a sigmoid activation due to the fact that we are predicting binary outcomes.

### Training the Model

Here we will see how to create the inputs for our model and how to train it.

Note that when training with GPU, I had to use the Theano backend as tensorflow was giving strange errors. Training using CPU presents no such issues.

#### Imports

We import ModelCheckpoint so that we can save our model as it makes progress. The IMDB dataset is found in `keras.datasets`

#### Create Inputs

We start off by defining the maximum number of words to be used, as well as the maximum length of any review.

Next we load the data and inspect its shape.

Let’s now zero-pad the sequences (reviews) and subset the data so that we can later create one hot representations of it without running out of memory.

Here comes the tricky part. Making one hot representations of the reviews takes some clever indexing since we are trying to index a 3D array with 2D array.

#### Create Checkpoint

We want to be able to save our model at the end of each epoch if an improvement was made. This enables reproducibility of results, and if something goes wrong, we can reload the most recent model without having to restart the whole training procedure all over again.

#### Fit the Data

We are almost done! All that remains is to instantiate the model and fit the data. The following function does just that.

## Results

I haven’t trained the model for long at this point, so the results aren’t noteworthy. Also, I haven’t yet investigated what the optimal number of parameters for the model is, so that remains to be seen. Furthermore, the memory limitation also puts an upper bound on the quality of results that can be obtained.

However, now that you know how to build such a model, you can apply it to other datasets that are less memory intensive and probably get some decent results.

Stay tuned for further developments!