“…Task-oriented dialogue system has been a longstanding studied topic (Williams and Young, 2007;Lee et al, 2009;Huang et al, 2020b) and can be integrated into many practical applications such as virtual assistant (Sun et al, 2016(Sun et al, , 2017. Traditionally, task-oriented dialogue systems are built in the pipeline approach, which consists of four essential components: natural language understanding (Chen et al, 2016), dialogue state tracking (Lee and Stent, 2016;Zhong et al, 2018;Wu et al, 2019a), policy learning (Su et al, 2016;Peng et al, 2018;Su et al, 2018) and natural language generation (Sharma et al, 2017;Chen et al, 2019;Huang et al, 2020a).…”