Generate: assisted decoding now uses generate for the assistant

[gante]

What does this PR do?

Subset of the original changes in #27979

"Reworks assisted candidate generation to call .generate(), instead of having its own custom generation loop. For most models this is nothing more than a nice abstraction. However, for models with a custom generate() function, this means the assistant model will now make use of it! (🤔 does this mean that DistilWhisper gets better numbers with this refactor?)"

The following tests were run locally and are passing:

1. RUN_SLOW=1 py.test tests/models/whisper/ -k speculative
2. py.test tests/ -k test_assisted

[HuggingFaceDocBuilderDev]

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[gante]

@amyeroberts the failing test is also failing in the daily CI (i.e. unrelated to this PR, as it doesn't depend on assisted generation), and I can't reproduce it on my end 🤔

[amyeroberts]

Thanks for the refactor and running the slow tests! Looks a lot cleaner ❤️

[gante]

@amyeroberts I can't merge due to the failing test (which is also failing on main 👀 ). Would you be able to merge?

[gante]

(the test is flaky 👉 #28035)

[amyeroberts]

@gante In this case we can merge :)

edit: note this was discussed offline as the reason for failing tests was identified and confirmed as independent from this PR