Split out CUDA RPC tests (#55695)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55695
In order to be able to run CUDA tests on their own (e.g., to avoid running CPU tests on GPU machines).
Done by moving test methods to a separate class (and sometimes introducing a "common" base class for utils), and then providing new entry points inside a `cuda/` subdirectory.
Test Plan: Checked they are run on Sandcastle.
Reviewed By: mrshenli
Differential Revision: D27618198
fbshipit-source-id: 8f671657f79c8ae115748ab7752fe0066705893b
