Fast Decoding in Sequence Models Using Discrete Latent Variables


Auto-regressive sequence models based on deep neural networks, such as RNNs, Wavenet and Transformer are the state of the art on many tasks. However, they lack parallelism and are thus slow for long sequences. RNNs lack parallelism both during training and decoding, while architectures like WaveNet and Transformer are much more parallel during training, but still lack parallelism during decoding.

We present a method to extend sequence models using discrete latent variables that makes decoding much more parallel. The main idea behind this approach is to first autoencode the target sequence into a shorter discrete latent sequence, which is generated auto-regressively, and finally decode the full sequence from this shorter latent sequence in a parallel manner. We verify that our method works on the task of neural machine translation, where our models are an order of magnitude faster than comparable auto-regressive models. We also introduce a new method for constructing discrete latent variables that allows us to obtain good BLEU scores.