Port softplus to structured (#58482)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/58482
Test Plan: Imported from OSS
Reviewed By: soulitzer
Differential Revision: D28571059
Pulled By: ezyang
fbshipit-source-id: a1065294f3c459e7c99aaed9edb09f88705f58e9
