¹Retrieval-based dialog is popular because posing dialog as response selection (Gunasekara et al., 2019) simplifies system design (Boussaha et al., 2019). Unlike modular or end-to-end task-oriented systems, retrieval-based ones do not rely on dedicated modules for language understanding, dialog management, and generation. They mitigate the requirements for explicit task-specific semantics hand-crafted by domain experts (Henderson et al., 2014; Mrkšić et al., 2015, 2017).