message: Ask users to set NCCL_DEBUG=WARN for NCCL errors
  The error string sometimes is not helpful, e.g. "unhandled cuda error"(tensorflow#36510).
  NCCL_DEBUG=WARN logs more information
diff: @@ -50,7 +50,8 @@ int NcclManager::instance_count = 0;
  do { \
  ncclResult_t nccl_status = (__VA_ARGS__); \
  if (nccl_status != ncclSuccess) { \
  - return errors::Internal("NCCL: ", ncclGetErrorString(nccl_status)); \
  + return errors::Internal("NCCL: ", ncclGetErrorString(nccl_status), \
  + ". Set NCCL_DEBUG=WARN for detail."); \
  } \
  } while (0)
  