added GPTNeoXForTokenClassification
# What does this PR do?

It adds the class GPTNeoXForTokenClassification, which allows using GPT NeoX models for token classification tasks. The implementation follows the one for other models (such as GPT2 and GPT Neo) closely and simply adds a linear layer after the hidden states.

## 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#start-contributing-pull-requests),
      Pull Request section?
- [ ] 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.
- [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?

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.

@ArthurZucker
@younesbelkada
_The documentation is not available anymore as the PR was closed or merged._<|||||>Ready for review, @ArthurZucker  and @younesbelkada  👍 