Added conv ai 2 (Again)
The original PR -> https://github.com/huggingface/datasets/pull/1383

Reason for creating again - 

The reason I had to create the PR again was due to the master rebasing issue. After rebasing the changes, all the previous commits got added to the branch. 
Looking perfect to me, need to rerun the tests
 Thanks, @tanmoyio. 
How do I rerun the tests? Should I change something or push a new commit? @rkc007 you don't need to rerun it, @lhoestq @yjernite will rerun it, as there are huge number of PRs in the queue it might take lil bit of time.  ive just re-run the tests Thank you @abhishekkrthakur. Can you please rerun it again? It seems something was broken in CI during the previous test. @lhoestq Sorry for the mess. I don't know why this keeps on happening. I tried step by step process of updating the PR but seems something is wrong. This happened for 2nd time with the same PR. Apologies for that. 

New PR -> https://github.com/huggingface/datasets/pull/1527
Also, I fixed everything in the new PR.