[`FA2`] Add flash attention for `GPT-Neo`
Author: susnato
Pull request: yes

# What does this PR do?

<!--
Congratulations! You've made it this far! You're not quite done yet though.

Once merged, your PR is going to appear in the release notes with the title you set, so make sure it's a great title that fully reflects the extent of your awesome contribution.

Then, please replace this with a description of the change and which issue is fixed (if applicable). Please also include relevant motivation and context. List any dependencies (if any) that are required for this change.

Once you're done, someone will review your PR shortly (see the section "Who can review?" below to tag some potential reviewers). They may suggest changes to make the code even better. If no one reviewed your PR after a week has passed, don't hesitate to post a new comment @-mentioning the same persons---sometimes notifications get lost.
-->

<!-- Remove if not applicable -->

Adds Flash Attention 2 for `GPT-Neo` as discussed in in this issue - https://github.com/huggingface/transformers/issues/26350 .


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [ ] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

<!-- Your PR will be replied to more quickly if you can figure out the right person to tag with @

 If you know how to use git blame, that is the easiest way, otherwise, here is a rough guide of **who to tag**.
 Please tag fewer than 3 people.

Models:

- text models: @ArthurZucker and @younesbelkada
- vision models: @amyeroberts
- speech models: @sanchit-gandhi
- graph models: @clefourrier

Library:

- flax: @sanchit-gandhi
- generate: @gante
- pipelines: @Narsil
- tensorflow: @gante and @Rocketknight1
- tokenizers: @ArthurZucker
- trainer: @muellerzr and @pacman100

Integrations:

- deepspeed: HF Trainer/Accelerate: @pacman100
- ray/raytune: @richardliaw, @amogkam
- Big Model Inference: @SunMarc
- quantization (bitsandbytes, autogpt): @SunMarc and @younesbelkada

Documentation: @stevhliu and @MKhalusova

HF projects:

- accelerate: [different repo](https://github.com/huggingface/accelerate)
- datasets: [different repo](https://github.com/huggingface/datasets)
- diffusers: [different repo](https://github.com/huggingface/diffusers)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

Maintained examples (not research project or legacy):

- Flax: @sanchit-gandhi
- PyTorch: See Models above and tag the person corresponding to the modality of the example.
- TensorFlow: @Rocketknight1

 -->

cc : @younesbelkada 

Comments:
Again, all tests are passing except the test_flash_attn_2_generate_use_cache test.
![Screenshot from 2023-09-29 14-18-40](https://github.com/huggingface/transformers/assets/56069179/b4629585-a32b-45ff-919b-1249da58f80f)

Hi @younesbelkada , you might need to run the tests on your end to double check, I am so sorry for the inconvenience.

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_26486). All of your documentation changes will be reflected on that endpoint.

I have updated the readme.

Hi @younesbelkada, @amyeroberts, I have made the necessary changes and this is ready for another review.

All the `flash attention` tests are passing.

![Screenshot from 2023-11-02 19-36-44](https://github.com/huggingface/transformers/assets/56069179/3e697474-17d6-43a3-bb3a-2943aec45484)


Hi @amyeroberts, I have removed all padding_masks.

Hi @amyeroberts, the conflict is resolved!

I think that `padding_mask`s should removed from the [startcoder](https://github.com/huggingface/transformers/blob/5964f820db1568d26298b37dea9db328185c7f7c/src/transformers/models/gpt_bigcode/modeling_gpt_bigcode.py#L243) too, since it was not an accepted input previously. WDYT?

@susnato If you could remove it from there too - that would be great! 

@amyeroberts , Ok I will open a separate PR after an hour!
BTW let me know if this PR needs any more changes.

@susnato Thanks! Nope - it's good to go. I'll merge :)