Fix comet metric
_The documentation is not available anymore as the PR was closed or merged._ Finally I'm done updating the dependencies ^^'

cc @sashavor can you review my changes in the metric card please ? Looks good to me! Just fixed a tiny typo :wink:  Thanks !
The COMET metric has been broken for a while since big breaking changes happened. We did not catch them in the CI because the slow test mocks the download_model function that was changed.

This PR fixes the metric, updates the download_model mock and updates the doctest.