[WIP] Inputs embeds for flax gpt neo
Author: mattf1n
Pull request: yes

# 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.

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Fixes #18036


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@sanchit-gandhi 

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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_18672). All of your documentation changes will be reflected on that endpoint.

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!

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.