they are both used to sync one tensor.

all_gather pytorch/distributed_c10d.py at 101a6263309ae2f9e52947c7d02d630e1190b6c3 · pytorch/pytorch · GitHub 7
all_gather_multigpu pytorch/distributed_c10d.py at 101a6263309ae2f9e52947c7d02d630e1190b6c3 · pytorch/pytorch · GitHub 7