Fix and refactor check_repo
# What does this PR do?

This PR fixes issues that a user may have with `make quality`/`make fixup` when all backends are not installed: in that case `requires_backends` is imported in the main init on top of the dummy objects and the script complains it's not documented.

The PR also refactors the white-list for the model that are not in an Auto-class, which was mostly containing Encoder and Decoder pieces of seq2seq models.