Recently, pre-trained language representation models such as GPT (Radford et al., 2018 (Radford et al., , 2019 , ELMo (Peters et al., 2018) , BERT<cite> (Devlin et al., 2019)</cite> and XLNet have achieved promising results in NLP tasks, including reading comprehension (Rajpurkar et al., 2016) , natural language inference (Bowman et al., 2015; Williams et al., 2018) and sentiment classification (Socher et al., 2013) .