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Fixes#3983
Mimicing what we do for saving and loading VectorDBQA chain, I added the
logic for RetrievalQA chain.
Also added a unit test. I did not find how we test other chains for
their saving and loading functionality, so I just added a file with one
test case. Let me know if there are recommended ways to test it.
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Co-authored-by: Harrison Chase <hw.chase.17@gmail.com>
