extract dispatch keys from optional Tensors (unboxed) (#58296)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/58296
Test Plan: Imported from OSS
Reviewed By: Chillee
Differential Revision: D28436822
Pulled By: bhosmer
fbshipit-source-id: 8031c9a3c121483dd0e5ed7b8b165952477108e4
