Remove decoder_input_ids from RAG dummy inputs
cc @ydshieh 

The old RAG dummy inputs didn't have `decoder_input_ids` but the new ones do - this seems like the most likely cause of the memory blowup, because RAG probably does a lot of weird retrieval stuff.
_The documentation is not available anymore as the PR was closed or merged._<|||||>unfortunately, the problematic situation (GPU usage) I described on Slack channel is still the same. It still takes 5 - 6 G extra memory at some point, compared to the commit before `814de8fa`.<|||||>Closing this because the dummies turned out not to be the problem after all!