Add Bert flash attention2
Author: sorenmc
Pull request: yes

# What does this PR do?

This introduces flash attention 2 for bert as discussed in  https://github.com/huggingface/transformers/issues/26350

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Fixes # (issue)


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- text models: @ArthurZucker and @younesbelkada
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Comments:
cc @younesbelkada 

Hi @younesbelkada 

No problem! I feel a little stuck with the CI errors I am getting. I finished most of the work for bert, but as you can see `check_repository_consistency` fails because several models are linked to BERT, meaning that i would have to do this same implementation for 10+ models. Might need some help with that, or figure out an alternative way to split some of these up into smaller tasks that could be included in a later PR. Also tests are failing for `Wav2Vec2` that i have not touched - I'm guessing this could be related to the consistency checks?






You should first rebase on main, then run `make fix-copies` that changes will be automatically ported to architectures that are similar to bert. But yes, this requires adding tests using `Copied from` as well in the testing files of all affected models! 

Unrelated test are fixed in main 🤗 

This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.