Allow RAG to output decoder cross-attentions
# What does this PR do?

This PR makes RAG output the generator model's decoder cross-attentions when `output_attentions=True`.

Motivation and context: before this PR, RAG's output objects had attributes for the generator's encoder self-attentions and decoder self-attentions, but no option for the encoder-decoder cross-attentions. So this simply allows cross-attentions to be extracted, as well as fixing a small bug where `output_attentions` wasn't being passed into the generator.

Fixes #9468


## 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?
- [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. Yes - #9468 
- [x] 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? - I don't believe any new tests are necessary. Existing tests pass.


## Who can review?
@patrickvonplaten, @lhoestq

@lhoestq Thanks for the suggestions! All the CI checks pass now.