Paperswithcode dataset mapping

[julien-c]

This is a continuation of huggingface/huggingface_hub#43, encoded directly inside dataset cards.

As discussed:

• paperswithcode_id: null when the dataset doesn't exist on paperswithcode's side.
• I've added this new key at the end of the yaml instead of ordering all keys alphabetically as pyyaml's default. No strong opinion on that one though

[julien-c]

messed up my branch, repushing

[lhoestq]

Thanks !

I added paperswithcode_id to our tags validator.

The CI still fails because many dataset cards are still missing some sections or have invalid tags. The errors are unrelated to this PR so let's merge this one.

[julien-c]

live mapping can be found at https://huggingface.co/api/pwc/datasets-mapping and will be kept up to date going forward