[`FA2`] Add flash attention for for `DistilBert`
Author: susnato
Pull request: yes

# What does this PR do?

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Adds Flash Attention 2 for `DistilBert` 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).
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      Pull Request section?
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      [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.

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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

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cc : @younesbelkada 


Comments:
This time all the tests are passed because there are no generative models.

![Screenshot from 2023-09-29 17-19-36](https://github.com/huggingface/transformers/assets/56069179/816c6565-8f15-4e37-a6e6-822bf279e92f)



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.

Hi @younesbelkada, @amyeroberts this PR is ready for review.

All the `flash attention` tests are passing.
![Screenshot from 2023-11-02 21-41-17](https://github.com/huggingface/transformers/assets/56069179/0df9585c-fda1-41c4-9f09-d5d08f4bf64d)



Hi @amyeroberts , done!

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_26489). All of your documentation changes will be reflected on that endpoint.