Add metrics usage examples and tests
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.