[PT] optional -> c10::optional (#47144)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/47144
Change `optional` to `c10::optional` to avoid conflicting with `std::optional` in files that happen to include both.
Test Plan: contbuild
Reviewed By: yinghai
Differential Revision: D24662515
fbshipit-source-id: 1e72fbc791d585e797a7239305ab5e3f82ddfec9
