Dialog Tasks

It looks like we don't have many dialog tasks, especially for response generation.

This [webpage](https://github.com/sebastianruder/NLP-progress/blob/master/english/dialogue.md) contains a list of dialog tasks we can consider. I think we should include at least some of the popular tasks.

My suggestion list:
Dialog act classification - Switchboard corpus
Dialog state tracking - Wizard-of-Oz, MultiWOZ
Dialog generation - PERSONA-CHAT, [DailyDilog++](https://iitmnlp.github.io/DailyDialog-plusplus/), [DSTC7-challenge](https://github.com/mgalley/DSTC7-End-to-End-Conversation-Modeling).