Implement ravel (#46098)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/46098
Doc:
Test Plan: Imported from OSS
Reviewed By: glaringlee
Differential Revision: D24253213
Pulled By: ejguan
fbshipit-source-id: 42a866c902272cbe3743a9d0cb3afb9165d51c0b
