Generate: Deprecate returning legacy cache by default; Handle use_cache=False

[gante]

What does this PR do?

Another step towards using Cache everywhere 💪

This PR makes the following [Cache+generate]-related changes:

1. Don't initialize a cache when use_cache=False (fixes Cache updating when use_cache = False #32843 )
2. generate tests now explicitly pass use_cache, instead of setting it in model.config 🤢 We were relying on a LOT of side effects, and missing the incorrect case mentioned in Cache updating when use_cache = False #32843
3. Add sanity-checks on cache-related parameters, in generation_config
4. Add a deprecation cycle on the default cache return type, so we start returning a Cache instance by default on generate
5. Isolate all cache initialization logic in generate into a single function, and reorganize the logic by blocks

[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]

moved up to this documentation section (Parameters that control the generation strategy used), which makes more sense

[gante]

new cache-related docs section in GenerationConfig, moved all cache-related flags here

[gante]

(>120 chars/line)

[gante]

(moved)

[gante]

In our docs we often mention that there are two ways to parameterize generate (generation_config or pass arg to generate). I don't think we need to be verbose here.

Also, setting through config is deprecated 😉

[gante]

rewrote this one.

The old description was outdated (legacy cache), and we now know that different models have different caches, so we shouldn't be precise here. The model class docs can be more precise, let's redirect users there.

[gante]

(These aliases made sense in the past, not anymore. They are, however, hard to deprecate!)

[gante]

New function, moving the cache logic from generate. I've organized the logic in blocks, putting the cases where we DON'T prepare a new cache at the top.

It is doing essentially the same, except for the Quick escape route 2, which is new. Added the warning in Quick escape route 3.

[gante]

This logic is expanded to handle a deprecation cycle

[gante]

changes in this file: pass use_cache to generate, instead of relying on model.config.use_cache=False and its side-effects

added a check to confirm that the cache is None when we pass use_cache=False

[ArthurZucker]

Cool! Let's make sure slow tests all pass as well here!

[ArthurZucker]

let's create an issue and leave it up to the community in the mean time!

[gante]

#32946