  Recurrent neural networks like long short-term memory (LSTM) are important
architectures for sequential prediction tasks. LSTMs (and RNNs in general)
model sequences along the forward time direction. Bidirectional LSTMs
(Bi-LSTMs) on the other hand model sequences along both forward and backward
directions and are generally known to perform better at such tasks because they
capture a richer representation of the data. In the training of Bi-LSTMs, the
forward and backward paths are learned independently. We propose a variant of
the Bi-LSTM architecture, which we call Variational Bi-LSTM, that creates a
channel between the two paths (during training, but which may be omitted during
inference); thus optimizing the two paths jointly. We arrive at this joint
objective for our model by minimizing a variational lower bound of the joint
likelihood of the data sequence. Our model acts as a regularizer and encourages
the two networks to inform each other in making their respective predictions
using distinct information. We perform ablation studies to better understand
the different components of our model and evaluate the method on various
benchmarks, showing state-of-the-art performance.
