Add CBT dataset
CONTRIBUTOR
Hi @lhoestq,

I have added changes from the review.
2021-03-17T10:06:39Z
https://github.com/huggingface/datasets/pull/2044#issuecomment-800957764
https://api.github.com/repos/huggingface/datasets/issues/2044
MDEyOklzc3VlQ29tbWVudDgwMDk1Nzc2NA==
https://api.github.com/repos/huggingface/datasets/issues/comments/800957764/reactions
2021-03-17T10:06:39Z
https://api.github.com/repos/huggingface/datasets/issues/comments/800957764
https://avatars.githubusercontent.com/u/29076344?v=4
https://api.github.com/users/gchhablani/events{/privacy}
https://api.github.com/users/gchhablani/followers
https://api.github.com/users/gchhablani/following{/other_user}
https://api.github.com/users/gchhablani/gists{/gist_id}
https://github.com/gchhablani
gchhablani
MDQ6VXNlcjI5MDc2MzQ0
https://api.github.com/users/gchhablani/orgs
https://api.github.com/users/gchhablani/received_events
https://api.github.com/users/gchhablani/repos
https://api.github.com/users/gchhablani/starred{/owner}{/repo}
https://api.github.com/users/gchhablani/subscriptions
User
https://api.github.com/users/gchhablani
public
CONTRIBUTOR
Thanks for approving @lhoestq 
2021-03-19T11:10:13Z
https://github.com/huggingface/datasets/pull/2044#issuecomment-802754090
https://api.github.com/repos/huggingface/datasets/issues/2044
MDEyOklzc3VlQ29tbWVudDgwMjc1NDA5MA==
https://api.github.com/repos/huggingface/datasets/issues/comments/802754090/reactions
2021-03-19T11:10:13Z
https://api.github.com/repos/huggingface/datasets/issues/comments/802754090
https://avatars.githubusercontent.com/u/29076344?v=4
https://api.github.com/users/gchhablani/events{/privacy}
https://api.github.com/users/gchhablani/followers
https://api.github.com/users/gchhablani/following{/other_user}
https://api.github.com/users/gchhablani/gists{/gist_id}
https://github.com/gchhablani
gchhablani
MDQ6VXNlcjI5MDc2MzQ0
https://api.github.com/users/gchhablani/orgs
https://api.github.com/users/gchhablani/received_events
https://api.github.com/users/gchhablani/repos
https://api.github.com/users/gchhablani/starred{/owner}{/repo}
https://api.github.com/users/gchhablani/subscriptions
User
https://api.github.com/users/gchhablani
public
This PR adds the [CBT Dataset](https://arxiv.org/abs/1511.02301).

Note that I have also added the `raw` dataset as a separate configuration. I couldn't find a suitable "task" for it in YAML tags.

The dummy files have one example each, as the examples are slightly big. For `raw` dataset, I just used top few lines, because they are entire books and would take up a lot of space.

Let me know in case of any issues.