Updated the RAG training with latest Pytorch Lightning library and the RAY
# What does this PR do?
Updated the RAG script with the latest libraries.


## 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.
- [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).
- [x] Did you write any new necessary tests? not applicable


## Who can review?
@patrickvonplaten 


_The documentation is not available anymore as the PR was closed or merged._<|||||>@shamanez are you planning to also update the end2end rag example? <|||||>> @shamanez are you planning to also update the end2end rag example?

Yes soon. I am also thinking of merging RAG with [Retro-like](https://deepmind.com/research/publications/2021/improving-language-models-by-retrieving-from-trillions-of-tokens) training.