Doc styler examples
# What does this PR do?

Last bit of the new style-doc script, blackify all code examples which:
- insures they don't have bacis Python errors
- are on par with our usual code format

This PR fixes a few code examples mistakes (see first commit for the docstrings and [this commit](https://github.com/huggingface/transformers/pull/14953/commits/999b9fd9f4a00da10353b6b95a4c842d0153da0b) for the MDX files) that were identified by black not being happy. Only code samples marked with `py` or `python` are blackified, so it's easy to turn off this feature if it becomes too annoying on some docstrings.
Cool! Good to merge for me once all tests pass