Add metrics usage examples and tests
{'login': 'lhoestq', 'id': 42851186, 'node_id': 'MDQ6VXNlcjQyODUxMTg2', 'avatar_url': 'https://avatars.githubusercontent.com/u/42851186?v=4', 'gravatar_id': '', 'url': 'https://api.github.com/users/lhoestq', 'html_url': 'https://github.com/lhoestq', 'followers_url': 'https://api.github.com/users/lhoestq/followers', 'following_url': 'https://api.github.com/users/lhoestq/following{/other_user}', 'gists_url': 'https://api.github.com/users/lhoestq/gists{/gist_id}', 'starred_url': 'https://api.github.com/users/lhoestq/starred{/owner}{/repo}', 'subscriptions_url': 'https://api.github.com/users/lhoestq/subscriptions', 'organizations_url': 'https://api.github.com/users/lhoestq/orgs', 'repos_url': 'https://api.github.com/users/lhoestq/repos', 'events_url': 'https://api.github.com/users/lhoestq/events{/privacy}', 'received_events_url': 'https://api.github.com/users/lhoestq/received_events', 'type': 'User', 'site_admin': False}
[]
[]
All metrics finally have usage examples and proper fast + slow tests :)

I added examples of usage for every metric, and I use doctest to make sure they all work as expected.

For "slow" metrics such as bert_score or bleurt which require to download + run a transformer model, the download + forward pass are only done in the slow test.
In the fast test on the other hand, the download + forward pass are monkey patched.

Metrics that need to be installed from github are not added to setup.py because it prevents uploading the `datasets` package to pypi.
An additional-test-requirements.txt file is used instead. This file also include `comet` in order to not have to resolve its *impossible* dependencies.

Also `comet` is not tested on windows because one of its dependencies (fairseq) can't be installed in the CI for some reason.
{'url': 'https://api.github.com/repos/huggingface/datasets/issues/1820/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/1820', 'html_url': 'https://github.com/huggingface/datasets/pull/1820', 'diff_url': 'https://github.com/huggingface/datasets/pull/1820.diff', 'patch_url': 'https://github.com/huggingface/datasets/pull/1820.patch', 'merged_at': '2021-02-05T14:00:00Z'}