updated the original RAG implementation to be compatible with latest Pytorch-Lightning
The original RAG version was not working with PL>=1.3, specially due to the fact that DDPAccelerator class has removed (Retriever Initialization of RAG).  The new version of PL library advises us to use DDP pluggings as a replacement.  

I also updated lightning_base.py regarding the new PL version. Now RAG works with the latest libraries. 


@patrickvonplaten  @lhoestq 

Hey @shamanez,

Could you run `make style`? 

@lhoestq - could you take a look as well?<|||||>Hey @patrickvonplaten 

I did run the "make style" and it changed following files and working alright.
<|||||>Thanks @patrickvonplaten  :)