support iteration tuple unpacking (#21985)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21985
ghimport-source-id: 1f20a8db7b6bad23b18ac1caefcb46b3fa141697
Test Plan: Imported from OSS
Differential Revision: D15948549
Pulled By: wanchaol
fbshipit-source-id: 758c9c3dfad40c4158aee21ddebcd25b711111d7
