Port softshrink to structured (#57623)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/57623
Test Plan: Imported from OSS
Reviewed By: VitalyFedyunin
Differential Revision: D28224703
Pulled By: ezyang
fbshipit-source-id: 62e40d53eb130205f6c4d2775082e436e6adadce
