NeurIPS 2019
Sun Dec 8th through Sat the 14th, 2019 at Vancouver Convention Center
Paper ID:1726
Title:Fast Structured Decoding for Sequence Models

Reviewer 1

The paper proposes to boost translation quality of a non-autoregressive (NART) neural machine translation system through a conditional random field (CRF) that is attached to the decoder. The CRF reduces the translation quality drop compared to autoregressive neural translation systems by imposing a bigram-language model like structure onto the decoder that helps to alleviate the strong independence assumption that NART architectures entail. The CRF is jointly trained with all other parameters of the neural network. Experiments conducted on WMT14 and IWSLT14 En-De and De-En tasks are reported to yield improvements of more than 6 BLEU points over their corresponding baselines. By augmenting the decoder with a Markov-order 1 CRF, the resulting network is strictly speaking no longer a non-autoregressive system. The CRF has similar qualities as using a bigram-language model, and even if the increase in latency at inference time is small, one may yield similar quality improvements with only marginal latency increase by choosing one of the many other autoregressive components as the last decoder layer. (Even collecting the bag of top-k scored tokens at each target sentence position and conducting a fast beam-search using a trigram language model may already give similar improvements.) The paper does not describe on which hardware the latency measurements were taken, and there is also no explanation of how the rescoring experiments reported in Table 2 were conducted. L34: The citations on previous studies is incomplete. E.g., the work by Jason Lee, Elman Mansimov and Kyunghyun Cho on "Deterministic Non-Autoregressive Neural Sequence Modeling by Iterative Refinement" is missing. L38: In Equation~(2), the variable z is unbound, and its explanation as "a well-designed input z" is insufficient for its understanding. L40: The examples shown in Table 1 refer to a specific NART model, and the authors present these examples as if any NART model would exhibit the same type of translation errors, which is clearly not the case. For better clarity, the authors should provide (or reference) a description of the specific architecture and hyper-parameter choice of their NART model from which they derived their examples. L44: The use of the terms "multimodality" and "multimodal distribution" seems inappropriate and somewhat of a misnomer in this context: There is no indication that the target distribution has more modes ("peaks") than can be captured by the network. The root cause merely seems to be the independence assumption. The same goes for L82 L83 and L204. Maybe the authors meant multinomial (?). L59: It's best to give a reference to those subsections that will describe the low-rank approximation and the beam approximation. L140: Z(x) is not a normalization *constant* but a normalization factor (aka. as partition function). L214 The approach to predict the target length T' merely as the source length T offset by a constant C seems implausible: Particularly for a language such as German, where compounds occur more frequently compared to English, one would expect a linear relationship between T and T'. Would it not be more plausible to make T' a constant and use padding tokens to fill up the target sentences? L222: The authors should make clear whether they compute case-sensitive or case-insensitive BLEU scores. Judging from the examples given in Table 1, I conject that a case-insensitive BLEU score evaluation was used. A case-insensitive evaluation artificially inflates BLEU scores though, whereas the baseline numbers in Table 2 that are cited from literature report case-sensitive BLEU scores and therefore tend to be nominally lower. The paper does not describe how the rescoring results reported in Table 2 were set up nor what the numbers 10, 100, 9, and 19 refer to.

Reviewer 2

The paper proposes to view MT as a sequence labeling problem modeled with a linear chain CRF, noting that such an approach becomes feasible if the length of the target output is fixed or predicted (as is common in non-autoregressive MT). The authors use a more or less standard transformer encoder-decoder architecture, but the decoder is non-autogressive and simply consumes a fixed number of padding tokens, and the log probability of the sequence is modeled with a CRF, which makes use of the transformer outputs at each time-step. Experimentally, the authors show that they can outperform many recent non-autoregressive MT baselines, while attaining comparable speedups. Originality: as noted above, this does appear to be an interesting and rather original idea, at least for neural MT. I think the main promise of this approach is in exact decoding, though the authors do not investigate this much. Quality and Clarity: Though the paper is easy to follow, I think the presentation could be improved in several respects: - I think it's a little strange to refer to the proposed method as non-autoregressive; it is autoregressive, though it uses only the previous label/token as its history. - Equations (2) and (3) should be corrected so there is no p(z|x), since z (which is the input of padding tokens) is not random and is not modeled. Similarly, if p(T'|x) is random (which it doesn't appear to be) the left-hand-sides should be changed to p(y, T' |x). - I think the discussion of the proposed method's runtime on lines 178-181 needs to be longer and perhaps formalized a bit more: in particular, the authors should justify why they view their proposed approach as being O(n k^2), and more pressingly, what they view the complexity to be of decoding under the models with which they compare. For instance, what do they view as the decoding complexity of an RNN-based decoder (perhaps with no attention)? Experimentally, the authors compare with a large number of recent baselines, which is very impressive. However, I believe some of the baselines could be improved. In particular, the Transformer baseline appears to use a beam size of 4, which will slow it down. It would be good to see its performance with a beam of size 1. Even better, training a Transformer model on the beam-searched outputs of a teacher Transformer model (i.e., with knowledge distilliation) can often lead to improved performance even with greedy decoding; note that this is the most fair comparison, since the non-autoregressive models are also trained from a teacher Transformer model. Furthermore, the authors do not include timing information for the RNN decoder, which should also be linear in the length of the output sequence. (Attention to the source complicates this a bit, though there are recent models (e.g., Press and Smith (2018)) that get good performance without it). Update: Thanks for your response. I'm increasing my score to a 7 in light of the response, especially given the distilled greedy Transformer results.

Reviewer 3

My main concern with this paper is authors calling their approach non-autoregressive. Running forward backward algo in CRF breaks that conditional independence assumption among predicted tokens which makes the proposed approach autoregressive. Although I would agree with authors that decoding with CRF is much more lightweight and is faster compared to decoding with autoregressive Transformer, where output has to be fed back in as an input into a deep sequence model. I would suggest authors using word "fast" neural machine translation instead of "non-autoregressive" neural machine translation in the paper. Regarding model itself, I would also like to see an ablation study on the effect of vanilla non-autoregressive loss as well use of different target lengths on the final performance in Table 2. Apart from my points above overall I believe it is a well executed paper that introduces several techniques to make CRF + Deep Neural Nets applicable for Neural Machine Translation and I recommend its acceptance.