add wit dataset
Resolves #2902 based on conversation there - would also close #2810. Open to suggestions/help 😀

CC @hassiahk @lhoestq @yjernite 
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. Finally got around to this PR, sorry for the delay!

I added some minimal info to the dataset card so we can at least have it on the hub and work more on it later.

Unfortunately, this re-triggered the tests and it looks like we need a rebase? cc @nateraw  Yea we need to rebase. Also I think it would be best to update the features to all be strings to match the original datasets format. I ran into a lot of issues with incorrect data types deep into the Dataset info Json process because it couldn't cast correctly. 

I mentioned this to @mariosasko - he might pick up this PR if I don't have bandwidth to finish soon.  > I think this is a clear occasion to start supporting "canonical" datasets:
> - without dataset_infos.json file
> - without dummy data

I agree we can support datasets without them. However the dataset_infos.json file can at least be **highly encouraged** since it has very nice info about the size of the dataset in MB and in number of examples, and it also helps guaranteeing the integrity of the dataset when it's downloaded (verification of the file checksums and the split sizes). This dataset now lives here: https://huggingface.co/datasets/wikimedia/wit_base. @nateraw @yjernite Thanks for the initial work.