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Title: Understanding Knowledge Distillation in Non-autoregressive Machine Translation

Paper content:
Non-autoregressive machine translation (NAT) systems predict a sequence of output tokens in parallel, achieving substantial improvements in generation speed compared to autoregressive models.
Existing NAT models usually rely on the technique of knowledge distillation, which creates the training data from a pretrained autoregressive model for better performance.
Knowledge distillation is empirically useful, leading to large gains in accuracy for NAT models, but the reason for this success has, as of yet, been unclear.
In this paper, we first design systematic experiments to investigate why knowledge distillation is crucial to NAT training.
We find that knowledge distillation can reduce the complexity of data sets and help NAT to model the variations in the output data.
Furthermore, a strong correlation is observed between the capacity of an NAT model and the optimal complexity of the distilled data for the best translation quality.
Based on these findings, we further propose several approaches that can alter the complexity of data sets to improve the performance of NAT models.
We achieve the state-of-the-art performance for the NAT-based models, and close the gap with the autoregressive baseline on WMT14 En-De benchmark.
Traditional neural machine translation (NMT) systems (Bahdanau et al., 2015; Gehring et al., 2017; Vaswani et al., 2017) generate sequences in an autoregressive fashion; each target token is predicted step-by-step by conditioning on the previous generated tokens in a monotonic (e.g. left-to-right) order.
While such autoregressive translation (AT) models have proven successful, the sequential dependence of decisions precludes taking full advantage of parallelism afforded by modern hardware (e.g. GPUs) at inference time.
On the other hand, there is a recent trend of non-autoregressive translation (NAT) models (Gu et al., 2018; , trading the model's capacity for decoding efficiency by making it possible predict the whole sequence or multi-token chunks of the sequence simultaneously.
Such a non-autoregressive factorization assumes that the output tokens are independent from each other.
However, this assumption obviously does not hold in reality and as a result NAT models generally perform worse than standard AT models.
One key ingredient to reducing the performance degradation of NAT models that is used in almost all existing works (Gu et al. (2018) ; ; Stern et al. (2019) , inter alia) is creation of training data through knowledge distillation (Hinton et al., 2015) .
More precisely, sequence-level knowledge distillation (Kim & Rush, 2016 ) -a special variant of the original approach -is applied during NAT model training by replacing the target side of training samples with the outputs from a pre-trained AT model trained on the same corpus with a roughly equal number of parameters.
It is usually assumed (Gu et al., 2018 ) that knowledge distillation's reduction of the "modes" (alternative translations for an input) in the training data is the key reason why distillation benefits NAT training.
However, this intuition has not been rigorously tested, leading to three important open questions:
• Exactly how does distillation reduce the "modes", and how we could we measure this reduction quantitatively?
Why does this reduction consistently improve NAT models?
• What is the relationship between the NAT model (student) and the AT model (teacher)?
Are different varieties of distilled data better for different NAT models?
• Due to distillation, the performance of NAT models is largely bounded by the choice of AT teacher.
Is there a way to further close the performance gap with standard AT models?
In this paper, we aim to answer the three questions above, improving understanding of knowledge distillation through empirical analysis over a variety of AT and NAT models.
Specifically, our contributions are as follows:
• We first visualize explicitly on a synthetic dataset how modes are reduced by distillation ( §3.1).
Inspired by the synthetic experiments, we further propose metrics for measuring complexity and faithfulness for a given training set.
Specifically, our metrics are the conditional entropy and KL-divergence of word translation based on an external alignment tool, and we show these are correlated with NAT model performance ( §3.2).
• We conduct a systematic analysis ( §4) over four AT teacher models and six NAT student models with various architectures on the standard WMT14 English-German translation benchmark.
These experiments find a strong correlation between the capacity of an NAT model and the optimal dataset complexity for the best translation quality.
• Inspired by these observations, we propose approaches to further adjust the complexity of the distilled data in order to match the model's capacity ( §5).
We also show that we can achieve the state-of-the-art performance for NAT and largely match the performance of the AT model.
In this paper, we first systematically examine why knowledge distillation improves the performance of NAT models.
We conducted extensive experiments with autoregressive teacher models of different capacity and a wide range of NAT models.
Furthermore, we defined metrics that can quantitatively measure the complexity of a parallel data set.
Empirically, we find that a higher-capacity NAT model requires a more complex distilled data to achieve better performance.
Accordingly, we propose several techniques that can adjust the complexity of a data set to match the capacity of an NAT model for better performance.
A EXPERIMENTAL DETAILS

assistant: text: We systematically examine why knowledge distillation is crucial to the training of non-autoregressive translation (NAT) models, and propose methods to further improve the distilled data to best match the capacity of an NAT model.