Raise error when missing or incorrect dates

[yonigozlan]

What does this PR do?

Running utils/add_dates.py in repo-consistency check doesn't raise errors, so missing dates are not correctly signaled. This PR checks all model cards and raise an error if any of them is missing a date string (very fast). Also checks if some transformers release dates are wrong (much slower, but only checks models included in the git diff in repo-consistency). Note that the command in repo-consistency only check for problems and explicitly tells contributors what command to run to fix the issues, but doesn't fix the model cards automatically.

The PR also adds a command in fix-copies to fix the errors on model cards included in the git diff.

Hopefully with that, we'll always have correct transformers release dates, and no model will be released without a transformers release date in its model card.

As a reminder, these dates are used in this space, which is included in the docs, so it would be nice to keep it accurate and up-to-date.

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[stevhliu]

Super nice, thanks!