Make sure to use return dict for the encoder call inside RagTokenForGeneration
## What does this PR do?

At some point, `return_dict` was set to be `False` by default inside BART. However, this created a type error inside `RagTokenForGeneration`, which was still written with the expectation that `return_dict=True` by default. This PR simply adds `return_dict=True` to the call to the BART encoder inside the `RagTokenForGeneration` code.

## Tests
I did not create new tests because this change is very minor. I did run all the existing tests and they pass.

## Who can review?

Anyone can review, but I tagged these two because this involves RAG and BART:

@patrickvonplaten @lhoestq

Thanks!