Generate: can_generate() recursive check

[gante]

What does this PR do?

The deprecation warning added in #33203 was incomplete: classes inheriting from classes that have received the update were throwing the warning. This PR adds a recursive check to can_generate() and tests the warnings thrown by the function.

Example of script that is throwing a warning, but shouldn't (and is fixed in this PR):

from transformers.models.llama import LlamaForCausalLM

class NewLlamaForCausalLM(LlamaForCausalLM):
    pass

model = NewLlamaForCausalLM.from_pretrained("hf-internal-testing/tiny-random-LlamaForCausalLM")
model.generate()

Thank you @regisss for warning me about this one and providing a reproducer 🙏

[HuggingFaceDocBuilderDev]

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[LysandreJik]

Ok! @regisss can you comment whether this fixes the issue you encountered?

[regisss]

Ok! @regisss can you comment whether this fixes the issue you encountered?

Yep it's all good on my side, the warning doesn't appear anymore with this fix. Thanks!

[ArthurZucker]

Thanks, wil have the same issue for great grand pa check no?

[ArthurZucker]

is this only grandfather check?

[gante]

it's recursive! (it calls can_generate of the parent, which in turn may call the granparent's, and so on :D )