Fix GPT-NeoX-20B past handling, attention computation
# What does this PR do?

* Fixes GPT-NeoX-20B handing of the past object to correctly be used in .generate
* Swaps attention computation for one more similar in the original training code, to hopefully avoid NaNs
* Update docstring, removed unnecessary dropout configs in config object

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Fixes # (issue)

https://github.com/huggingface/transformers/issues/17632 
https://github.com/huggingface/transformers/issues/17452 (Hopefully)

## Before submitting
- [x] 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?
- [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.
- [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).
- [ ] Did you write any new necessary tests?


## Who can review?

@sgugger
_The documentation is not available anymore as the PR was closed or merged._<|||||>There are a few equivalence tests failing with the PR, if you can dive into it. Let us know if you need any help!<|||||>I've run the tests locally and they pass, so I can't seem to reproduce the test errors. Can someone else give them a try?<|||||>The tests pass on GPU but not on CPU on my side. So doing
```
CUDA_VISIBLE_DEVICES="" pytest tests/models/gpt_neox/test_modeling_gpt_neox.py
```
reproduces the failure.<|||||>Thanks again! Nice to be able to use GPT-Neo-X in float16 for generations :-)