Make the big table creation/check platform independent
# What does this PR do?

As @jplu mentioned in #8813, the check that the big table of models/tokenziers is up-to-date (done in `make quality`) requires all three backends installed (plus tokenizers and sentencepiece). This PR amends the script to use the objects in the init (that are always there, thanks to the dummies) instead of the dicts in the auto modules (which are set to None if a specific backend is not installed).

In passing, it adds aliases `MT5Tokenizer` and `MT5TokenizerFast` (to `T5Tokenizer` and `T5TokenizerFast` respectively) because otherwise the script does not detect the tokenizers associated to this model, cc @patrickvonplaten 