Add guards for USE_C10D_FOO in relevant c10d files (#59697)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/59697
The c10d build process selectively adds files based on the `USE_C10D_FOO` flags (where `FOO` is one of `GLOO`, `NCCL` or `MPI`). Replicating this logic inside libtorch will be harder, since libtorch uses a simpler approach (i.e., it lists the files in `build_variables.bzl`). So instead we could always include all files, and "disable" each file as needed using `#ifdef`s. Note that this is not a new approach: we already do the same for all the files of the TensorPipe agent based on the flag `USE_TENSORPIPE`.
ghstack-source-id: 131169540
Test Plan: CI
Reviewed By: agolynski
Differential Revision: D28987577
fbshipit-source-id: 4c6195de4e9a58101dad9379537e8d055dfd38af
