trunk/e632bee7b8c7e590b70bf70283dde89dc451c855: [c10d] Make nccl2 the default NCCL backend (#192281)

发生了什么

The standard nccl backend still selected legacy ProcessGroupNCCL unless users opted into nccl2 through an environment variable, even though the explicit nccl2 and nccl-legacy names make backend selection reversible. Invert the registration so an unset variable or TORCH_DIST_USE_NCCL2=1 selects nccl2, while TORCH_DIST_USE_NCCL2=0 and the explicit nccl-legacy name remain rollback paths. Defaulting also exposed three n…

摘要按规则整理自下方来源原文

来源