  Non-autoregressive neural machine translation (NAT) usually employs
sequence-level knowledge distillation using autoregressive neural machine
translation (AT) as its teacher model. However, a NAT model often outputs
shorter sentences than an AT model. In this work, we propose sequence-level
knowledge distillation (SKD) using perturbed length-aware positional encoding
and apply it to a student model, the Levenshtein Transformer. Our method
outperformed a standard Levenshtein Transformer by 2.5 points in bilingual
evaluation understudy (BLEU) at maximum in a WMT14 German to English
translation. The NAT model output longer sentences than the baseline NAT
models.
