Add instructional error message for cudnn RNN double backward workaround (#33884)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/33884
Mitigates https://github.com/pytorch/pytorch/issues/5261.
It's not possible for us to support cudnn RNN double backwards due to
limitations in the cudnn API. This PR makes it so that we raise an error
message if users try to get the double backward on a cudnn RNN; in the
error message we suggest using the non-cudnn RNN.
Test Plan: - added some tests to check the error message
Reviewed By: albanD
Differential Revision: D20143544
Pulled By: zou3519
fbshipit-source-id: c2e49b3d8bdb9b34b561f006150e4c7551a78fac
