an inappropriate root number in nccl test

[tonyyang-svail]

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

Paddle/paddle/operators/nccl_op_test.cu.cc

Lines 242 to 245 in 15e8c80

	TEST_F(NCCLTester, ncclBcastOp) {
	std::unique_ptr<f::OpDesc> op2(new f::OpDesc);
	const int kRoot = 5;
	op2->SetType("ncclBcast");

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]