Created by: tonyyang-svail

We have kRoot set to 5 at in NCCLTester.ncclBcastOp.

https://github.com/PaddlePaddle/Paddle/blob/15e8c80ee0cba5c26d881b955afc66e59aaccbdb/paddle/operators/nccl_op_test.cu.cc#L242-L245

However, this implies the machine to have at least 6 GPUs. If it is not the case, we will get an invalid argument error. invalid argument at [/paddle/paddle/operators/nccl_op.cu.cc:193]