Generate: unify LogitsWarper and LogitsProcessor

[gante]

What does this PR do?

LogitsProcessor and LogitsWarper were originally created as two separate classes, with LogitsWarper being a LogitsProcessor that should only be applied to sample-based decoding methods. In practice, they are exactly the same.

Having two separate classes meant that there was undesired code duplication: two functions to instantiate these objects, two separate pieces of logic to apply these processors, etc.

⚠️ Perhaps more importantly, given that the order of the logit processors is important, having two separate types of objects results in needing custom logic if we want to interleave the two types of instances. Since we don't want custom logic, this PR is a requirement for an ongoing project integration.

This PR:

• removes all internal uses of LogitsWarper and related logic, moving it to the logic related to LogitsProcessor
• keeps LogitsWarper public with a deprecation warning, for BC

[HuggingFaceDocBuilderDev]

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[LysandreJik]

Indeed, that's much cleaner! Thanks for the cleanup @gante