Add clipping to relative positional embedding
# What does this PR do?

This PR adds clipping to distance embedding as described in the paper [Improve Transformer Models with Better Relative Position Embeddings](https://arxiv.org/abs/2009.13658)
Without this addition, the model with relative positional embedding will return an error if the input length above max_position_embeddings param.


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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      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.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?
@patrickvonplaten @LysandreJik @julien-c @zhiheng-huang<|||||>Hey @hadaev8,

Thanks for your PR. It would be awesome if you could provide a code snippet to this PR that shows a case where the current implementation of BERT's relative positional embeddings would fail. Hope it's fine to tag the original author here: @zhiheng-huang for some discussion. <|||||>@patrickvonplaten 
Colab notebook
https://colab.research.google.com/drive/1bAwwNMbh27JW0H6uWG9rZgyjz_b0eB1M?usp=sharing
<|||||>@patrickvonplaten 
Any update?<|||||>Hey @hadaev8,

I'm having a hard time deciding here without the opinion of the official author @zhiheng-huang . @zhiheng-huang it would be great if you could leave a comment here :-)<|||||>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/master/CONTRIBUTING.md) are likely to be ignored.<|||||>@patrickvonplaten 
Well, in current state model with rel pos just would not work as expected eg process longer input.<|||||>@hadaev8, 

Alright, let's try to fix it together! It would be awesome if you could post a reproducible code snippet here so that we can see when an error arises<|||||>@patrickvonplaten 
Here it doesnt work
https://colab.research.google.com/drive/1bAwwNMbh27JW0H6uWG9rZgyjz_b0eB1M?usp=sharing
Here it work
https://colab.research.google.com/drive/1OIHjR2kVndDzwro7n_SgkpJG4YwjgGQY?usp=sharing<|||||>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/master/CONTRIBUTING.md) are likely to be ignored.