Add sgn to torch.rst so that it appears in the built docs (#51479)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51479
Fixes https://github.com/pytorch/pytorch/issues/50146
Test Plan: Imported from OSS
Reviewed By: mruberry
Differential Revision: D26179734
Pulled By: anjali411
fbshipit-source-id: 1cda9a3dc9ce600e585900eea70fbecac0635d5c
