Port softplus to structured (#57620)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/57620
Test Plan: Imported from OSS
Reviewed By: VitalyFedyunin
Differential Revision: D28224705
Pulled By: ezyang
fbshipit-source-id: a48419f5958e4d29427fb1fec7ff929f0297e4e4
