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Title: STCN: Stochastic Temporal Convolutional Networks

Paper content:
Convolutional architectures have recently been shown to be competitive on many
sequence modelling tasks when compared to the de-facto standard of recurrent neural networks (RNNs) while providing computational and modelling advantages due to inherent parallelism.
However, currently, there remains a performance
gap to more expressive stochastic RNN variants, especially those with several layers of dependent random variables.
In this work, we propose stochastic temporal convolutional networks (STCNs), a novel architecture that combines the computational advantages of temporal convolutional networks (TCN) with the representational power and robustness of stochastic latent spaces.
In particular, we propose a hierarchy of stochastic latent variables that captures temporal dependencies at different time-scales.
The architecture is modular and flexible due to the decoupling of the deterministic and stochastic layers.
We show that the proposed architecture achieves state of the art log-likelihoods across several tasks.
Finally, the model is capable of predicting high-quality synthetic samples over a long-range temporal horizon in modelling of handwritten text.
Generative modeling of sequence data requires capturing long-term dependencies and learning of correlations between output variables at the same time-step.
Recurrent neural networks (RNNs) and its variants have been very successful in a vast number of problem domains which rely on sequential data.
Recent work in audio synthesis, language modeling and machine translation tasks BID8 BID9 BID13 has demonstrated that temporal convolutional networks (TCNs) can also achieve at least competitive performance without relying on recurrence, and hence reducing the computational cost for training.Both RNNs and TCNs model the joint probability distribution over sequences by decomposing the distribution over discrete time-steps.
In other words, such models are trained to predict the next step, given all previous time-steps.
RNNs are able to model long-term dependencies by propagating information through their deterministic hidden state, acting as an internal memory.
In contrast, TCNs leverage large receptive fields by stacking many dilated convolutions, allowing them to model even longer time scales up to the entire sequence length.
It is noteworthy that there is no explicit temporal dependency between the model outputs and hence the computations can be performed in parallel.
The TCN architecture also introduces a temporal hierarchy: the upper layers have access to longer input sub-sequences and learn representations at a larger time scale.
The local information from the lower layers is propagated through the hierarchy by means of residual and skip connections BID2 .However
, while TCN architectures have been shown to perform similar or better than standard recurrent architectures on particular tasks BID2 , there currently remains a performance gap to more recent stochastic RNN variants BID3 BID7 BID11 BID12 BID14 BID25 . Following
a similar approach to stochastic RNNs, BID21 present a significant improvement in the log-likelihood when a TCN model is coupled with latent variables, albeit at the cost of limited receptive field size. The computational
graph of generative (left) and inference (right) models of STCN. The approximate posterior
q is conditioned on dt and is updated by the prior p which is conditioned on the TCN representations of the previous time-step dt−1. The random latent variables
at the upper layers have access to a long history while lower layers receive inputs from more recent time steps.In this work we propose a new approach for augmenting TCNs with random latent variables, that decouples deterministic and stochastic structures yet leverages the increased modeling capacity efficiently. Motivated by the simplicity
and computational advantages of TCNs and the robustness and performance of stochastic RNNs, we introduce stochastic temporal convolutional networks (STCN) by incorporating a hierarchy of stochastic latent variables into TCNs which enables learning of representations at many timescales. However, due to the absence
of an internal state in TCNs, introducing latent random variables analogously to stochastic RNNs is not feasible. Furthermore, defining conditional
random variables across time-steps would result in breaking the parallelism of TCNs and is hence undesirable.In STCN the latent random variables are arranged in correspondence to the temporal hierarchy of the TCN blocks, effectively distributing them over the various timescales (see FIG0 . Crucially, our hierarchical latent
structure is designed to be a modular add-on for any temporal convolutional network architecture. Separating the deterministic and stochastic
layers allows us to build STCNs without requiring modifications to the base TCN architecture, and hence retains the scalability of TCNs with respect to the receptive field. This conditioning of the latent random variables
via different timescales is especially effective in the case of TCNs. We show this experimentally by replacing the TCN
layers with stacked LSTM cells, leading to reduced performance compared to STCN.We propose two different inference networks. In the canonical configuration, samples from each
latent variable are passed down from layer to layer and only one sample from the lowest layer is used to condition the prediction of the output. In the second configuration, called STCN-dense, we
take inspiration from recent CNN architectures BID18 and utilize samples from all latent random variables via concatenation before computing the final prediction.Our contributions can thus be summarized as: 1) We present a modular and scalable approach to augment
temporal convolutional network models with effective stochastic latent variables. 2) We empirically show that the STCN-dense design prevents
the model from ignoring latent variables in the upper layers BID32 . 3) We achieve state-of-the-art log-likelihood performance,
measured by ELBO, on the IAM-OnDB, Deepwriting, TIMIT and the Blizzard datasets. 4) Finally we show that the quality of the synthetic samples
matches the significant quantitative improvements.
In this paper we proposed STCNs, a novel auto-regressive model, combining the computational benefits of convolutional architectures and expressiveness of hierarchical stochastic latent spaces.
We have shown the effectivness of the approach across several sequence modelling tasks and datasets.
The proposed models are trained via optimization of the ELBO objective.
Tighter lower bounds such as IWAE BID5 or FIVO (Maddison et al., 2017) may further improve modeling performance.
We leave this for future work.
The network architecture of the proposed model is illustrated in FIG6 .
We make only a small modification to the vanilla Wavenet architecture.
Instead of using skip connections from Wavenet blocks, we only use the latent sample zt in order to make a prediction of xt.
In STCN-dense configuration, zt is the concatenation of all latent variables in the hierarchy, i.e., zt = [z Output layer f (o) : For the IAM-OnDB and Deepwriting datasets we use 1D convolutions with ReLU nonlinearity.
We stack 5 of these layers with 256 filters and filter size
1. DISPLAYFORM0 For TIMIT and Blizzard datasets Wavenet blocks in the output layer perform significantly better.
We stack 5 Wavenet blocks with dilation size
1. For each convolution operation in the block we use 256 filters.
The filter size of the dilated convolution is set to
2. The STCN-dense-large model is constructed by using 512 filters instead of 256.

assistant: text: We combine the computational advantages of temporal convolutional architectures with the expressiveness of stochastic latent variables.