examples/rag: test coverage, tiny model

[sshleifer]

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

[stas00]

sign me up, Sam

[shamanez]

@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.

[stas00]

I think this is still waiting for: #8284 to complete the missing info and perhaps some tests were added since then?

[github-actions]

This issue has been stale for 1 month.

[stas00]

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...

[4imothy]

Should this issue be closed looks like rag now has some tests here and here.