[GPT2] Correct gradient checkpointing
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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Previously, it was not possible to train GPT2 with gradient_checkpointing and `use_cache=False`. However `use_cache` should not be set to `True` when training. This PR corrects the behavior so that
gradient checkpointing does not require `use_cache=True`.

In addition, this PR changes lists to tuples in GPT2 for consistency with other models.


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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- [ ] Did you write any new necessary tests?


## Who can review?

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 albert, bert, XLM: @LysandreJik
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