[`GPTNeoX`] Flex Attention + Refactor
Author: vasqu
Pull request: yes

# What does this PR do?
Adds flex attention and the refactor according to #34809

However, I discovered several issues in the current version of gemma2 (#34282):
- It seems like that flex attention needs a transpose afterwards like sdpa
- Loading flex attn with from pretrained didn't work and hence, current tests use another attn implementation (eager or sdpa not sure again)
- Tests could gain from similar tests like sdpa :D for now it's a bit of a hassle to always have some integration test added when it could be a more general test for all subsequent models
- I'm not familiar with better transformers or limitations of flex attn --> added some todos in case we need to check in
- Flex attn doesn't support dropout (or maybe I've overlooked something)
- Setting `model.config._attn_implementation = ...` should be tracked somewhere and checked for sanity as done the first time - for now it silently overwrites and could cause some ugly errors (tested with changing to flash attention 2 while not having fa2 installed)
- Documentation should be added somewhere (prolly perf or something else)

So tbh, I'm not sure whether to split this PR into several ones, e.g. a gemma fix, general loading, general tests, docs, and then subsequent models, or not

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@ArthurZucker 

Comments:
CI failings seem unrelated, flaky tests (e.g. XLM, Qwen2VL)

Possible TODO -> fallback to eager when using head mask in fa2, sdpa + add head mask in flex attention (should be possible via score mod)

Edit: Added now

Yes I was testing it for Gemma as well it needed a transpose at the end as well 

If you don't mind could you check the pull request i did for gemma seems I keep failing some tests 

Also the gemma 2 now supports new stuff in the  configuration which confused me a lot
The attn logit soft capping  


 Also the model.config._attn_implementation is not really implemented correctly for example it does not  correctly uses the correct attn upon choosing one


Still working on the gemma flex attention pr might help with the docs as well


@dame-cell I'll take a look tomorrow! I'm busted for today :) 

But as quick thing to let the loading be handled correctly look into my changes into the utils folder and modeling_utils. With those changes, loading should be handled correctly. Tbh, that's one of the main reasons why I think it might be better to split some PRs and get loading etc correctly first before we start adding.

Edit: One last thing to change would be to add `_supports_flex_attn = True` then like done for sdpa, fa2

> @dame-cell I'll take a look tomorrow! I'm busted for today :) 
> 
> But as quick thing to let the loading be handled correctly look into my changes into the utils folder and modeling_utils. With those changes, loading should be handled correctly. Tbh, that's one of the main reasons why I think it might be better to split some PRs and get loading etc correctly first before we start adding.

Hmmm ohh I get it I see thanks for letting me know 😀

Feel free to ping me once you feel like this is ready! 🤗 

I think it should be ready @ArthurZucker just found something on the fly just a min ago, should be good to go :smile: 

Edit: the ci failure doesn't seem related to this PR

I'll take a look tomorrow or the day after :) 

@ArthurZucker updated per review, looking forward to the next round ;)

@ArthurZucker Hopefully the last round :crossed_fingers: