Nezha Pytorch implementation
# What does this PR do?

This PR adds a pytorch implementation of the NEZHA model to transformers. [NEZHA](https://arxiv.org/abs/1909.00204) was introduced by Huawei Noah's Ark Lab in late 2019 and it is widely used in the Chinese NLP community. This implementation is based on the official pytorch implementation of NEZHA and the current BERT pytorch implementation . The model checkpoints are also from the [official implementation](https://github.com/huawei-noah/Pretrained-Language-Model/tree/master/NEZHA-TensorFlow). 

Fixes # (issue)


## 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?
- [ ] 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.

Since the model is quite similar to bert, maybe @LysandreJik?
_The documentation is not available anymore as the PR was closed or merged._<|||||>ready for review! I'll upload the rest of the pre-trained models later today<|||||>addressed all the comments from @sgugger and uploaded the two remaining models. Ready for a final round of review.<|||||>Thanks again for your contribution!