[BE] move init_multigpu_helper to common_distributed (#67050)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/67050
This PR moves init_multi_gpu_helper to common_distributed so that it could be shared by different distributed tests.
ghstack-source-id: 141370119
Test Plan: wait for ci.
Reviewed By: mrshenli
Differential Revision: D31842644
fbshipit-source-id: c7bad25d6cef9bdce7ad1fb6c60c1cad4b765702
