[nnc] Added an implementation of sign op (#64033)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/64033
Test Plan: Imported from OSS
Reviewed By: bertmaher
Differential Revision: D30579197
Pulled By: navahgar
fbshipit-source-id: f9f7fa7f2ffa109cf4e441eb1af821b8b891d4d3
