Allow passing encoder_ouputs as tuple to EncoderDecoder Models
# What does this PR do?

Fixes #15536

For now I have added a test of the functionality to an existing test, since I'm not sure if there should be a separate test (or a test at all) for such a minor issue. Let me know if there is a better way.

## 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?
- [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.
https://github.com/huggingface/transformers/issues/15536
- [ ] 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?

@patrickvonplaten 
_The documentation is not available anymore as the PR was closed or merged._<|||||>
@jsnfly , thanks you for this PR... Is it possible to do this fix for a T5 model as well.. It is also a sequence to sequence models and sometime we may want to pass a tuple to the decoder.