Add WIT Dataset
closes #2981 #2810

@nateraw @hassiahk I've listed you guys as co-author as you've contributed previously to this dataset
_The documentation is not available anymore as the PR was closed or merged._ Hi! Coming in late with some context.

There are two versions of the WIT dataset:
1. The original source dataset managed by Wikimedia. It has more information, raw image representations, and each row corresponds to an image linked to all of its captions wherever it happens in Wikipedia (in multiple languages)
2. The Google version, corresponding to the data script in this PR, which duplicates image instances and requires the user to download the images themselves from the provided URL (note that a basic implementation will have them download the same picture several time. @thomasw21 using our download manager instead of `urllib` could help with that, but it wouldn't be required if people had access to the first version)

The Wikimedia folks were really interested in us hosting a ready-to-go streaming version of this dataset where users don't have to download the version themselves, which is why we have the pre-processed versions on an HF bucket, with the raw images and a pre-computed embedding (don't remember the model, we can keep it ). That's the data script currently in https://github.com/huggingface/datasets/pull/2981 . It's nearly ready to go, the one thing we should do is move the raw data from our HF google Cloud bucket to the Hub.

How do you want to move forward? IMO the best way would be to have a WIT dataset under the Wikimedia org with both configurations, but it depends on everyone's timelines Okay after offline discussion. We'll improve this versions and push it to the hub under `google` namespace. 

> which duplicates image instances and requires the user to download the images themselves from the provided URL (note that a basic implementation will have them download the same picture several time. @thomasw21 using our download manager instead of urllib could help with that, but it wouldn't be required if people had access to the first version)

Ah interesting wasn't aware of this duplication issue, concretely it'll just mean that our dataset in bigger than expected ... I think this should be handled after this loading script (though I have to figure our how to spawn a dl_manager).

> The Wikimedia folks were really interested in us hosting a ready-to-go streaming version of this dataset where users don't have to download the version themselves, which is why we have the pre-processed versions on an HF bucket, with the raw images and a pre-computed embedding (don't remember the model, we can keep it ). That's the data script currently in https://github.com/huggingface/datasets/pull/2981 . It's nearly ready to go, the one thing we should do is move the raw data from our HF google Cloud bucket to the Hub.

Similarly a script will be written and pushed to `wikimedia` organisation. @mariosasko can you make one last review concerning the text description changes? Then I'll handle putting it under `google` namespace and close this PR. Looks all good now. Great job!  Closing as this has been migrated to the hub under `google` namespace: https://huggingface.co/datasets/google/wit