CI: avoid human error, automatically infer generative models

[gante]

What does this PR do?

This PR:

1. Replaces manual definition of generative models to test with generate with an automatic definition -- if model_class.can_generate(), then it runs tests from GenerationTesterMixin. No more human mistakes, which happened frequently in the past months 🐛 🔫
2. Now that we run generation tests for all models that can generate, there are a few (old) bad apples. Explicitly skip them i.e. overwrite the automated all_generative_model_classes and explain why certain classes are being skipped. (bad apples detected with py.test tests/models/ -k test_greedy_generate -vv) 💔
3. Moves tests that call generate from the generic test mixin to GenerationTesterMixin. This means a) we can have a better overview of what's being tested with generate; and b) model architectures without generative capabilities will have fewer skips 🎯

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In a follow-up PR:

1. We need to manually define the model's main input name (e.g., pixel_values) in the model tester. Make it use model.main_input_name instead, to avoid human error Done ✅
2. Despite the changes in this PR, generate tests will only run if GenerationTesterMixin is inherited. We can easily forget to add the mixin, resulting in a false positive. Add an automated check: if any of the model classes can generate, then GenerationTesterMixin must be inherited in the tester

[HuggingFaceDocBuilderDev]

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[zucchini-nlp]

Super cool! If VLMs start failing on generation tests, that's okay. I have a local draft for that, which needs to be reviewed after a few other PRs are merged