Get rid of torch._thnn. (#25354)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25354
It doesn't seem to be used anymore.
Test Plan: Imported from OSS
Reviewed By: ezyang
Differential Revision: D17101577
Pulled By: gchanan
fbshipit-source-id: b7c00de8c05bff1336d2012fd7b6f97709391e17
