more complete tuple assignments (#21949)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21949
ghimport-source-id: 458793d74af3728bf0338867b081157905a7635a
Test Plan: Imported from OSS
Differential Revision: D15948550
Pulled By: wanchaol
fbshipit-source-id: 9ed69e0859e052816f06fc9c288b905551b2e48c
