[WIP] Inputs embeds for flax gpt neo

[mattf1n]

What does this PR do?

Adds the option to pass inputs_embeds to FlaxGPTNeoForCausalLM. This is already an option for the PyTorch version of the model GPTNeoForCausalLM.

Fixes #18036

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, Pull Request section?
• [ ] Was this discussed/approved via a Github issue or the forum? 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, and here are tips on formatting docstrings.
• [ ] 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.

@sanchit-gandhi

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint.

[sanchit-gandhi]

Hey @mattf1n - let me know if you're still interested in completing this PR and I'd be happy to help with any questions/queries!

[github-actions[bot]]

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