Make beam_constraints.Constraint.advance() docstring more accurate

[alex-calderwood]

What does this PR do?

The abstract base class Constraint.advance() docstring indicated that it could only return an integer token_id, when in fact it can return a list of token_ids.

Fixes #32668
#32668

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@gante @stevhliu

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[stevhliu]

Thanks for the fix! We can merge once @gante approves as well 🙂

[gante]

Yup, that's correct, thank you for fixing 🤗