Make Gloo an optional c10d dependency (#22257)
Summary:
The CMake modifications include removal of some unnecessary paths
(e.g. find_package(CUDA) and friends) that are no longer used since
c10d is always part of the larger torch build. The macro
`C10D_USE_...` was ambiguous and is now removed in favor of only
having top level `USE_...`. The c10d test suite is changed to include
skip annotations for the tests that depend on Gloo as well.
Now, if you compile with `USE_DISTRIBUTED=1` and `USE_GLOO=0` you get
a functioning build for which the tests actually pass.
Closes https://github.com/pytorch/pytorch/issues/18851.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/22257
Differential Revision: D16087993
Pulled By: pietern
fbshipit-source-id: 0cea66bd5cbd9736b06fa1d45ee13a18cab88adb
