Add StableAdamW Optimizer
Author: capemox
Pull request: yes

# What does this PR do?
This PR adds the `StableAdamW` optimizer from the [`optimi`](https://optimi.benjaminwarner.dev/) package. In this PR:
- StableAdamW is added to `OptimizerNames`
- The optimizer can be initialized through `TrainingArguments`, gets imported and used in `Trainer`
- Test cases are written to verify the proper behaviour of the model under `TrainerIntegrationTest`
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Fixes #36564 


## 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?
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      to it if that's the case.
- [x] 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).
- [x] Did you write any new necessary tests?


## Who can review?
@muellerzr @SunMarc should be the right people to review this!

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Models:

- text models: @ArthurZucker
- vision models: @amyeroberts, @qubvel
- speech models: @eustlb
- graph models: @clefourrier

Library:

- flax: @gante and @Rocketknight1
- generate: @zucchini-nlp (visual-language models) or @gante (all others)
- pipelines: @Rocketknight1
- tensorflow: @gante and @Rocketknight1
- tokenizers: @ArthurZucker
- trainer: @muellerzr and @SunMarc
- chat templates: @Rocketknight1

Integrations:

- deepspeed: HF Trainer/Accelerate: @muellerzr
- ray/raytune: @richardliaw, @amogkam
- Big Model Inference: @SunMarc
- quantization (bitsandbytes, autogpt): @SunMarc @MekkCyber

Documentation: @stevhliu

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: @Rocketknight1
- PyTorch: See Models above and tag the person corresponding to the modality of the example.
- TensorFlow: @Rocketknight1

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Comments:
Hi 👋, thank you for opening this pull request! The pull request is converted to draft by default. When it is ready for review, please click the `Ready for review` button (at the bottom of the PR page).

cc @muellerzr @SunMarc for review (but feel free to push the review off onto someone else if you think they're better suited!)

@SunMarc, I've updated the docs. Thanks for the approval!

Some of the tests are failing, which are what is remaining to finish the merge of this PR into main. What is the process to solve for the pending tests?

@SunMarc For the tests, it looks like the workflow `run_tests` is failing here: `tests/models/opt/test_modeling_opt.py::OPTModelTest::test_pipeline_text_classification` which generates `Failed: Timeout >120.0s` for `hf_hub_download()` .  Maybe increase the timeout to give more time to download? This is the remaining test to pass to be able to merge into main. Thank you @SunMarc !

I've created this PR to fix the github actions that is broken here https://github.com/huggingface/transformers/pull/39446. So i'm closing this PR !