add wit dataset
Opening this up - Here's whats left to make it really shine:
- [ ] Update dataset card (this is being worked on by the author)
- [ ] Add `dataset_info.json`. Requires downloading the entire dataset. I believe @lhoestq mentioned he may have a machine he's using for this sort of thing.

I think both of these _could_ be done in separate PRs, but I'm not picky either way.

Also, we may find that users (and/or the authors) would like additional configurations for languages, image only, embeddings only, etc. I think we can wait and see what the community says on this and address in different PR.
Resolves #2902 based on conversation there - would also close #2810. Open to suggestions/help 😀

CC @hassiahk @lhoestq @yjernite 