add for in string support (#21990)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21990
ghimport-source-id: 69b4882f8602c4088e7a833c43fd3cd37501a3c0
Test Plan: Imported from OSS
Differential Revision: D15948547
Pulled By: wanchaol
fbshipit-source-id: 057e7f4fb67c6dca98458ceb14414368e1a86260
