Create UCC ProcessGroup when ucc_lib available (#69564)
Summary:
This PR is based on https://github.com/pytorch/pytorch/pull/69552, please review that PR first. This PR requires https://github.com/facebookresearch/torch_ucc/pull/57, but this PR can be landed separately 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/69564
Reviewed By: mingzhe09088
Differential Revision: D35457276
Pulled By: jiayisuse
fbshipit-source-id: 662c5a771c7cfc92ab42955c9abd24e56e5cafda
(cherry picked from commit 985b5f86845ec616083379d6e623a40213293dcf)
