[Cache] rename max_batch_size -> batch_size in compilable caches

[gante]

What does this PR do?

WIP, see this comment

---

Uses deprecate_kwarg to rename max_batch_size to batch_size in all compilable caches. max_batch_size is a bad arg name: it implies that batch sizes smaller than max_batch_size can use the cache too, which is not the case.

---

Note that this deprecation was started before, but we messed it up along the way:

1. Deprecation process started: Cache: use batch_size instead of max_batch_size #32657
2. Deprecation message got changed to the opposite of the goal: Offloaded cache: fix generate #34921
3. User-contributed PR that respected the (modified) deprecation message: Remove deprecated batch_size parameter #37007

[github-actions]

Hi 👋, thank you for opening this pull request! The pull request is converted to draft by default. The CI will be paused while the PR is in draft mode. When it is ready for review, please click the Ready for review button (at the bottom of the PR page). This will assign reviewers and trigger CI.

[zucchini-nlp]

Oh wow, I even forgot we were deprecating the other way. Thanks for digging into it!

Since we talk about batch sizes, I remember this issue (#35444) where user wanted to contribute an actual max_batch_size (especially for enc-dec model cases). Similar to seq length, unused batches are all zeros. I think the feature is nice to have, but I also see we can mess up with users who manipulate directly cache._key_cache. WDYT about it, is it worth supporting?

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[gante]

@zucchini-nlp I see, the export use cache with a batch size > input batch size makes sense (export once, reuse with any batch size).

It's feasible, the questions are a) code complexity; b) throughput. I'm going to give it a go 🤞

[zucchini-nlp]

Yeah, I am also concerned if it will add too much complexity. Thanks, will be cool if it's doable with minimal maintenance cost :)

[gante]

It's actually quite clean and doesn't seem to have throughput disadvantages 👀 closing this PR in favor of expanding capabilities