introduce new --gen-dir flag to generate_code and use it in fbcode (#75800)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/75800
This leads to more similarities between OSS CMake and eventually OSS
Bazel. We will be able to generate files with the same names and not
have different file lists between the builds.
ghstack-source-id: 155300043
Test Plan: Verified locally and in CI.
Reviewed By: dreiss
Differential Revision: D35648586
fbshipit-source-id: 9f1638b5665ebcc64466883f65ef24a2bfd05228
(cherry picked from commit 7f2acff1baa8dfafddefdc720714f8d39feda436)
