Fix Inefficient GELU implementation in GPT2
Author: null-pointer-access
Pull request: yes

# What does this PR do?

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Fixes #39073 by using fused GELU instead of the custom implementation. This can improve the e2e efficiency by up to 12%.


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#create-a-pull-request),
      Pull Request section?
- [x] 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. #39073 
- [ ] 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.

cc @ArthurZucker 

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- text models: @ArthurZucker
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- accelerate: [different repo](https://github.com/huggingface/accelerate)
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Comments:
**[For maintainers]** Suggested jobs to run (before merge)

run-slow: gpt2