Make ProcessGroupNCCL load torch_ucc.so when TORCH_UCC_LIBRARY_PATH is set (#69552)
Summary:
This is the very first step for the UCC-NCCL integration. This PR lets `ProcessGroupNCCL` load the `torch_ucc.so` if the user specifies an environmental variable `TORCH_UCC_LIBRARY_PATH`. If this environment variable is not specified by the user, then there will be no visible change.
In the future, we may want to make PyTorch smart enough to automatically detect the `torch_ucc.so` in the user's system, but before doing that, I believe we should first make sure that `ProcessGroupUCC` is very well tested.
Note that in this PR, `ProcessGroupNCCL` just loads the library but will not use it. I am trying to make PRs small, so the usage of `torch_ucc.so` will be submitted in later PRs.
This PR requires the change in https://github.com/facebookresearch/torch_ucc/pull/56, otherwise `torch_ucc.so` can not be successfully loaded. But his PR can be landed separately without waiting for https://github.com/facebookresearch/torch_ucc/pull/56 because, in PyTorch's unit tests, UCC is never used or tested.
cc pietern mrshenli pritamdamania87 zhaojuanmao satgera rohan-varma gqchen aazzolini osalpekar jiayisuse SciPioneer H-Huang
Pull Request resolved: https://github.com/pytorch/pytorch/pull/69552
Reviewed By: mruberry
Differential Revision: D34675212
Pulled By: jiayisuse
fbshipit-source-id: a3d1fb98340dbe3a931af555423863efd381f1ae
(cherry picked from commit 3778b6fabe70c26b5a65e6ddec641d2ef9113cd1)
