Label Attention Network for Structured Prediction

Sequence labeling assigns a label to each token in a sequence, which is a fundamental problem in natural language processing (NLP). Many NLP tasks, including part-of-speech tagging and named entity recognition, can be solved in a form of sequence labeling problem. Other tasks such as constituency parsing and non-autoregressive machine translation can also be transformed into sequence labeling tasks. Neural models have been shown powerful for sequence labeling by employing a multi-layer sequence encoding network. Conditional random field (CRF) is proposed to enrich information over label sequences, yet it suffers large computational complexity and over-reliance on Marko assumption. To this end, we propose label attention network (LAN) to hierarchically refine representation of marginal label distributions bottom-up, enabling higher layers to learn more informed label sequence distribution based on information from lower layers. We demonstrate the effectiveness of LAN through extensive experiments on various NLP tasks including POS tagging, NER, CCG supertagging, constituency parsing and non-autoregressive machine translation. Empirical results show that LAN not only improves the overall tagging accuracy with similar number of parameters, but also significantly speeds up the training and testing compared to CRF.