Generate: models with custom generate() return True in can_generate()

[gante]

What does this PR do?

See discussion in #25532

This PR updates GenerationMixin.can_generate() to also return True when a model has a custom generate() -- which means that the model is allowed to call generate(). While at it, removes a few overloads, as they are redundant with the change in the PR ✂️

Additional context: ALL models have a generate() function defined, as the mixin is added to all models. This function is a helper to detect when the model class is compatible with generate() or not.

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Double-checking with code:

from transformers import Blip2ForConditionalGeneration, BertForQuestionAnswering, BarkFineModel, BarkModel, SpeechT5ForTextToSpeech

print(Blip2ForConditionalGeneration.can_generate())
print(BarkFineModel.can_generate())
print(BarkModel.can_generate())
print(SpeechT5ForTextToSpeech.can_generate())
print(BertForQuestionAnswering.can_generate())  # sanity check: this one can't generate

After this PR, this prints True in all cases except the last one, as expected.

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[amyeroberts]

Thanks for the update and tidying this up ❤️