[core] remove GenerationMixin inheritance by default in PreTrainedModel

[gante]

What does this PR do?

In v4.45, we set in motion the removal of GenerationMixin inheritance by default in PreTrainedModel, our base model class. You can recap the full list of reasons in the original PR, but TL;DR removes circular dependencies and make non-generative models more lightweight.

This PR is the final step: removes the GenerationMixin inheritance. Note that this change is NOT breaking in most contexts:
✅ Loading generate-capable transformers models (#33203 added direct inheritance, #36180 added a meta test to ensure we always add generate tests to generate-capable models)
✅ Loading non generate-capable models
✅ Loading generate-capable Hub models with AutoModelXXX (#33203 added the logic to ensure we add GenerationMixin even if the original model is missing it, as well as corresponding tests)
❌ Loading generate-capable Hub models directly, i.e. not using AutoModelXXX, if and only if the model doesn't inherit from GenerationMixin (= old implementation). In this case, an informative warning is thrown, suggesting to load the model with AutoModelXXX. This warning was present since v4.45.

Relevant tests:

• py.test tests/models -k test_generation_tester_mixin_inheritance
• py.test tests/models/auto/ -k test_custom_model_patched_generation_inheritance
• py.test tests/utils/test_modeling_utils.py::ModelUtilsTest::test_can_generate

---

After merging, let's keep an eye on issues. Although I think I've got the only breaking case well documented, Hub code is always a wildcard.

[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.

[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]

SpeechT5ForSpeechToText probably had some generate compatibility issues in the last versions, since its prepare_inputs_for_generation was not being updated

(removing the global mixin triggered test failures)

[ArthurZucker]

Have not fully followed but sounds good yep 🤗 remember we had some issues at the time, all stable now!