allow separate relative attention bias
# What does this PR do?

It supports umt5 models, which need separate relative attention biases for each layer.
The current code will have backward compatibility with previous T5 and MT5 checkpoints.

Fixes # (issue)
https://github.com/huggingface/transformers/issues/22573

## 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.
- [ ] 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?

Models:

- text models: @ArthurZucker and @younesbelkada 
- @stefan-it
_The documentation is not available anymore as the PR was closed or merged._<|||||>> Thanks for your PR. Transformers is not a modular toolbox so we never add functionality to existing models like this. If you need this parameter for a new model, you should create a copy of T5 with the add-new-model-like command and just adapt the modeling code.
> 
> Or you can jsut host your slightly modified T5 model on the Hub with the [code on the Hub API](https://huggingface.co/docs/transformers/custom_models).

Thanks for your feedback. No problem, I will close this PR and add another one that have a separate model.