Revert "Change ATEN generator argument type to const std::optional<Generator>& (#120076)"
This reverts commit 4305c64fea154ee1ab566e19bd7568753fc30916.
Reverted https://github.com/pytorch/pytorch/pull/120076 on behalf of https://github.com/izaitsevfb due to breaking internal builds(take 3) ([comment](https://github.com/pytorch/pytorch/pull/120076#issuecomment-1986338164))
