Simplify operator `sign` using the helper.
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/25592
Test Plan: Imported from OSS
Differential Revision: D17552470
Pulled By: VitalyFedyunin
fbshipit-source-id: 6c8cc4f46dd390c231b2d0aac664ad2a6ac8876e
