[`Llama`] Fix QnA loading from pretrained via loading hooks
Author: vasqu
Pull request: yes

# What does this PR do?
Currently, `LlamaForQuestionAnswering` is broken when we try to load from a pretrained ckpt, see #30381 for reference. In the same issue, a workaround has been suggested but I think this is too obscure and unintuitive. #29258 was supposed to fix general loading issues, but it hasn't. Due to BC reasons, I opted for loading hooks that imitate the base model prefix as in every other `ForXXX` class and rename it after successfully loading the model. This way, people that already have the transformer prefix can use it as usual and loading from a pretrained ckpt will work as expected. 

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Fixes #30381 (this time really :D)


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Comments:
Oops, might be not as great of a solution as I thought, I'll check later if I can fix the failing tests.