[pytorch] Remove c10::nullopt_t::init (#47013)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/47013
It was getting used in client code, and it's not part of `std::optional`.
ghstack-source-id: 115769682
Test Plan: Existing tests
Reviewed By: smessmer
Differential Revision: D24547710
fbshipit-source-id: a24e0fd03aba1cd996c85b12bb5dcdb3e7af46b5
