Fix WMT dataset loading issue and docs update
{'login': 'khushmeeet', 'id': 8711912, 'node_id': 'MDQ6VXNlcjg3MTE5MTI=', 'avatar_url': 'https://avatars.githubusercontent.com/u/8711912?v=4', 'gravatar_id': '', 'url': 'https://api.github.com/users/khushmeeet', 'html_url': 'https://github.com/khushmeeet', 'followers_url': 'https://api.github.com/users/khushmeeet/followers', 'following_url': 'https://api.github.com/users/khushmeeet/following{/other_user}', 'gists_url': 'https://api.github.com/users/khushmeeet/gists{/gist_id}', 'starred_url': 'https://api.github.com/users/khushmeeet/starred{/owner}{/repo}', 'subscriptions_url': 'https://api.github.com/users/khushmeeet/subscriptions', 'organizations_url': 'https://api.github.com/users/khushmeeet/orgs', 'repos_url': 'https://api.github.com/users/khushmeeet/repos', 'events_url': 'https://api.github.com/users/khushmeeet/events{/privacy}', 'received_events_url': 'https://api.github.com/users/khushmeeet/received_events', 'type': 'User', 'site_admin': False}
[]
[]
The PR branch now has some commits unrelated to the changes, probably due to rebasing. Can you please close this PR and open a new one from a new branch? You can use  `git cherry-pick` to preserve the relevant changes:
```bash
git checkout master
git remote add upstream git@github.com:huggingface/datasets.git
git pull --ff-only upstream master
git checkout -b wmt-datasets-fix2
git cherry-pick f2d6c995d5153131168f64fc60fe33a7813739a4 a9fdead5f435aeb88c237600be28eb8d4fde4c55
``` Closing this PR due to unwanted commit changes. Will be opening new PR for the same issue.
This PR is a fix for #4354 

Changes are made for `wmt14`, `wmt15`, `wmt16`, `wmt17`, `wmt18`, `wmt19` and `wmt_t2t`. And READMEs are updated for the corresponding datasets.

As I am on a M1 Mac, I am not able to create a virtual `dev` environment using `pip install -e ".[dev]"`. Issue is with `tensorflow-text` not supported on M1s and there is no supporting repo by Apple or Google. So, if I was needed to perform local testing, I am not able to do that.

Let me know, if any additional changes are required.

Thanks
{'url': 'https://api.github.com/repos/huggingface/datasets/issues/4537/reactions', 'total_count': 0, '+1': 0, '-1': 0, 'laugh': 0, 'hooray': 0, 'confused': 0, 'heart': 0, 'rocket': 0, 'eyes': 0}
{'url': 'https://api.github.com/repos/huggingface/datasets/pulls/4537', 'html_url': 'https://github.com/huggingface/datasets/pull/4537', 'diff_url': 'https://github.com/huggingface/datasets/pull/4537.diff', 'patch_url': 'https://github.com/huggingface/datasets/pull/4537.patch', 'merged_at': None}