stride is wrong when computing local var in the smem-layernorm.cu

[cailun01]

Hi, Arya!
I think it should be int stride = blockDim.x/2.

cuda/layernorm/kernels/smem-layernorm.cu

Line 52 in 18ee1d1

for(int stride = blockDim.x; stride > 0; stride /= 2){