Configurable Relative Position Max. Distance
# What does this PR do?

The relative attention on the T5 model can only configured with the number of buckets.
However, it has fixed max distance and can't be configured.

The current fixed max distance is working fine for text modality, but for much longer sequence use-cases, it has to be adjusted.

This pull request make it possible to configure the max distance in the relative attention.

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Fixes # (issue)


## 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/master/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.
- [ ] 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?


## Who can review?

@patrickvonplaten @sgugger  @patil-suraj @LysandreJik


_The documentation is not available anymore as the PR was closed or merged._