Optimizing away the `fill-mask` pipeline.
Author: Narsil
Pull request: yes

# What does this PR do?

- Don't send anything to the tokenizer unless needed. Vocab check is much
faster
- Keep BC by sending data to the tokenizer when needed. User handling
warning messages will see performance benefits again
- Make `targets` and `top_k` work together better `top_k` cannot be higher
than `len(targets)` but can be smaller still.
- Actually simplify the `target_ids` in case of duplicate (it can happen 
because we're parsing raw strings)
- Removed useless code to fail on empty strings. It works only if empty 
string is in first position, moved to ignoring them instead.
- Changed the related tests as only the tests would fail correctly
(having incorrect value in first position)



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Fixes #12099


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Comments:
Ping @LysandreJik Do you mind doing a quick review ?

(Tests do not have to be modified, for this to work, but it will output a lot of warning and be slower than necessary)

Thanks for the ping, reviewing now!

That's a very neat idea !

It must be quite slow though, right ?

Yes, definitely too slow to actually put in tests and generally a bad idea to rely on model hub checkpoints for this I think, it was just the quickest way to ensure that all tokenizer/model pairs really do continue working

Yes, maybe have a script or something for larger refactors for sure.