examples/rag: test coverage, tiny model
Disclaimer: I don't know this code very well, this may be much harder than it seems.
Blocking PR: #7713 


[`examples/rag/finetune.py`, `examples/rag/finetune.sh`, `eval_rag.py`] do not seem to be tested at all.
It would be good to have a `test_finetune.py` like `examples/seq2seq` that tested these.

cc @stas00 if interested, rag is a cool new retrieval model https://arxiv.org/pdf/2005.11401.pdf
sign me up, Sam<|||||>@sshleifer 

examples/rag/finetune.py is not that stable. Seems like it depends on the pytorch_lightning version also. It would be nice if we can test it properly. <|||||>I think this is still waiting for: https://github.com/huggingface/transformers/issues/8284 to complete the missing info and perhaps some tests were added since then?
<|||||>This issue has been stale for 1 month.<|||||>As the required to implement this info was never provided and I since then moved to work on other things I removed self-assignment to this ticket...