viable/strict/1789545173: [cuda][nccl] Drop the dead `NCCL_ALLTOALL_SUPPORTED` arm (#196400)

发生了什么

Summary: NCCL_ALLTOALL_SUPPORTED is not defined anywhere in the repo. D115674668 ("[ncclx][fix-build] Remove NCCL_ALLTOALL_SUPPORTED macro") deleted the #define from nccl.h.in in both v2_29 and v2_30 , specifically so that an old PyTorch built against a new NCCLX would stop selecting the capital- T ncclAllToAll API. So the defined(NCCL_ALLTOALL_SUPPORTED) term in all2all_single_equal_split is now always 0 , and the…

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