Add FewRel Dataset
Hi,

This PR closes this [Card](https://github.com/huggingface/datasets/projects/1#card-53285184) and Issue #1757.

I wasn't sure how to add `pid2name` along with the dataset so I added it as a separate configuration. For each (head, tail, tokens) triplet, I have created one example. I have added the dictionary key as `"relation"` in the dataset. Additionally, for `pubmed_unsupervised`, I kept `"relation":""` in the dictionary.

Please recommend better alternatives, if any.

Thanks,
Gunjan