Adds return type annotation for fork_rng function (#63724)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/63723
Since it's a generator function the type annotation shall be `Generator`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63724
Reviewed By: iramazanli
Differential Revision: D30543098
Pulled By: heitorschueroff
fbshipit-source-id: ebdd34749defe1e26c899146786a0357ab4b4b9b
