Adds return type annotation for fork_rng function (#63724)

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`.
![image](https://user-images.githubusercontent.com/47299190/130318830-29ef9529-0daa-463c-90b2-1b11f63ade8a.png)

Pull Request resolved: https://github.com/pytorch/pytorch/pull/63724

Reviewed By: iramazanli

Differential Revision: D30543098

Pulled By: heitorschueroff

fbshipit-source-id: ebdd34749defe1e26c899146786a0357ab4b4b9b