replace Generator arguments with c10::optional<Generator> (#36232)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/36232
The purpose of this PR is to replace `at::Generator generator = nullptr` with `c10::optional<at::Generator> = c10::nullopt` all over the code
* #36230 Replace std::shared_ptr with c10::intrusive_ptr in at::Generator
Test Plan: Imported from OSS
Differential Revision: D20943603
Pulled By: pbelevich
fbshipit-source-id: 65d335990f01fcc706867d5344e73793fad68ae6
