Make barrier blocking in UCC (#86961)
Currently CUDA UCC barrier is nonblocking with respect to CPU and there is no flag to change it. To make UCC PG barrier behaviour consistent with NCCL PG in this PR barrier has changed to be always blocking.
cc @mrshenli @pritamdamania87 @zhaojuanmao @satgera @rohan-varma @gqchen @aazzolini @osalpekar @jiayisuse @H-Huang @kwen2501 @awgu
Pull Request resolved: https://github.com/pytorch/pytorch/pull/86961
Approved by: https://github.com/kwen2501
