Add optional string support to native_functions schema (#43010)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/43010
Test Plan: Imported from OSS
Reviewed By: ngimel
Differential Revision: D23751851
Pulled By: mruberry
fbshipit-source-id: 648f7430e1b7311eff28421f38e01f52d998fcbd
