Fix RNN nonlinearity (#28058)
Summary:
This was referenced in the `RNN` docs but wasn't actually assigned
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28058
Pulled By: driazati
Differential Revision: D17945867
fbshipit-source-id: 0f0dc2633183a7e67a12352a2a7ac0545284666a
