Learning Knowledge-Enhanced Contextual Language Representations for Domain Natural Language Understanding

Knowledge-Enhanced Pre-trained Language Models (KEPLMs) improve the performance of various downstream NLP tasks by injecting knowledge facts from large-scale Knowledge Graphs (KGs). However, existing methods for pre-training KEPLMs with relational triples are difficult to be adapted to close domains due to the lack of sufficient domain graph semantics. In this paper, we propose a Knowledgeenhanced lANGuAge Representation learning framework for various clOsed dOmains (KAN-GAROO) via capturing the implicit graph structure among the entities. Specifically, since the entity coverage rates of closed-domain KGs can be relatively low and may exhibit the global sparsity phenomenon for knowledge injection, we consider not only the shallow relational representations of triples but also the hyperbolic embeddings of deep hierarchical entityclass structures for effective knowledge fusion. Moreover, as two closed-domain entities under the same entity-class often have locally dense neighbor subgraphs counted by max point biconnected component, we further propose a data augmentation strategy based on contrastive learning over subgraphs to construct hard negative samples of higher quality. It makes the underlying KELPMs better distinguish the semantics of these neighboring entities to further complement the global semantic sparsity. In the experiments, we evaluate KANGAROO over various knowledge-aware and general NLP tasks in both full and few-shot learning settings, outperforming various KEPLM training paradigms performance in closed-domains significantly. 1 * T. Zhang and R. Xu contributed equally to this work. † Co-corresponding authors. 1 All the codes and model checkpoints have been released to public in the EasyNLP framework (Wang et al., 2022) . URL: https://github.com/alibaba/EasyNLP . 2 The detailed analysis of entity coverage ratios and max point biconnected component is described in Sec. 2 MedKG Closed Domain CN-DBpedia The symptoms of COVID-19 in humans include respiratory infections and fever. Pre-training Corpus 15.73% 48.43% Entity Coverage Ratio 41.48% 25.37% Entity Max Point Biconnected Comp.