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Title: Twin Networks: Matching the Future for Sequence Generation

Paper content:
We propose a simple technique for encouraging generative RNNs to plan ahead.
We train a ``backward'' recurrent network to generate a given sequence in reverse order, and we encourage states of the forward model to predict cotemporal states of the backward model.
The backward network is used only during training, and plays no role during sampling or inference.
We hypothesize that our approach eases modeling of long-term dependencies by implicitly forcing the forward states to hold information about the longer-term future (as contained in the backward states).
We show empirically that our approach achieves 9% relative improvement for a speech recognition task, and achieves significant improvement on a COCO caption generation task.
Recurrent Neural Networks (RNNs) are the basis of state-of-art models for generating sequential data such as text and speech.
RNNs are trained to generate sequences by predicting one output at a time given all previous ones, and excel at the task through their capacity to remember past information well beyond classical n-gram models BID6 BID27 .
More recently, RNNs have also found success when applied to conditional generation tasks such as speech-to-text BID9 , image captioning BID61 and machine translation .RNNs
are usually trained by teacher forcing: at each point in a given sequence, the RNN is optimized to predict the next token given all preceding tokens. This
corresponds to optimizing one-stepahead prediction. As there
is no explicit bias toward planning in the training objective, the model may prefer to focus on the most recent tokens instead of capturing subtle long-term dependencies that could contribute to global coherence. Local correlations
are usually stronger than long-term dependencies and thus end up dominating the learning signal. The consequence is
that samples from RNNs tend to exhibit local coherence but lack meaningful global structure. This difficulty in
capturing long-term dependencies has been noted and discussed in several seminal works (Hochreiter, 1991; BID6 BID27 BID45 .Recent efforts to address
this problem have involved augmenting RNNs with external memory BID14 BID18 BID22 , with unitary or hierarchical architectures BID0 BID51 , or with explicit planning mechanisms BID23 . Parallel efforts aim to prevent
overfitting on strong local correlations by regularizing the states of the network, by applying dropout or penalizing various statistics BID41 BID64 BID15 BID32 BID39 . Figure 1: The forward and the backward
networks predict the sequence s = {x 1 , ..., x 4 } independently. The penalty matches the forward (or a
parametric function of the forward) and the backward hidden states. The forward network receives the gradient
signal from the log-likelihood objective as well as L t between states that predict the same token. The backward network is trained only by maximizing
the data log-likelihood. During the evaluation part of the network colored
with orange is discarded. The cost L t is either a Euclidean distance or a
learned metric ||g(h DISPLAYFORM0 with an affine transformation g. Best viewed in color.In this paper, we propose TwinNet
, 1 a simple method for regularizing a recurrent neural network that encourages modeling those aspects of the past that are predictive of the long-term future. Succinctly, this is achieved as follows: in parallel
to the standard forward RNN, we run a "twin" backward RNN (with no parameter sharing) that predicts the sequence in reverse, and we encourage the hidden state of the forward network to be close to that of the backward network used to predict the same token. Intuitively, this forces the forward network to focus
on the past information that is useful to predicting a specific token and that is also present in and useful to the backward network, coming from the future (Fig. 1) .In practice, our model introduces a regularization term
to the training loss. This is distinct from other regularization methods that
act on the hidden states either by injecting noise BID32 or by penalizing their norm BID31 BID39 , because we formulate explicit auxiliary targets for the forward hidden states: namely, the backward hidden states. The activation regularizer (AR) proposed by BID39 , which
penalizes the norm of the hidden states, is equivalent to the TwinNet approach with the backward states set to zero. Overall, our model is driven by the intuition (a) that the
backward hidden states contain a summary of the
future of the sequence, and (b) that in order to predict the future more accurately, the
model will have to form a better representation of the past. We demonstrate the effectiveness of the TwinNet approach experimentally
, through several conditional and unconditional generation tasks that include speech recognition, image captioning, language modelling, and sequential image generation. To summarize, the contributions of this work are as follows:• We introduce
a simple method for training generative recurrent networks that regularizes the hidden states of the network to anticipate future states (see Section 2);• The paper provides extensive evaluation of the proposed model on multiple tasks and concludes that it helps training and regularization for conditioned generation (speech recognition, image captioning) and for the unconditioned case (sequential MNIST, language modelling, see Section 4);• For deeper analysis we visualize the introduced cost and observe that it negatively correlates with the word frequency (more surprising words have higher cost).
In this paper, we presented a simple recurrent neural network model that has two separate networks running in opposite directions during training.
Our model is motivated by the fact that states of the forward model should be predictive of the entire future sequence.
This may be hard to obtain by optimizing one-step ahead predictions.
The backward path is discarded during the sampling and evaluation process, which makes the sampling process efficient.
Empirical results show that the proposed method performs well on conditional generation for several tasks.
The analysis reveals an interpretable behaviour of the proposed loss.One of the shortcomings of the proposed approach is that the training process doubles the computation needed for the baseline (due to the backward network training).
However, since the backward network is discarded during sampling, the sampling or inference process has the exact same computation steps as the baseline.
This makes our approach applicable to models that requires expensive sampling steps, such as PixelRNNs BID44 and WaveNet (Oord et al., 2016a) .
One of future work directions is to test whether it could help in conditional speech synthesis using WaveNet.We observed that the proposed approach yield minor improvements when applied to language modelling with PennTree bank.
We hypothesize that this may be linked to the amount of entropy of the target distribution.
In these high-entropy cases, at any time-step in the sequence, the distribution of backward states may be highly multi-modal (many possible futures may be equally likely for the same past).
One way of overcoming this problem would be to replace the proposed L2 loss (which implicitly assumes a unimodal distribution of the backward states) by a more expressive loss obtained by either employing an inference network BID30 or distribution matching techniques BID17 .
We leave that for future investigation.

assistant: text: The paper introduces a method of training generative recurrent networks that helps to plan ahead. We run a second RNN in a reverse direction and make a soft constraint between cotemporal forward and backward states.