add sync

Author: chengduoZH

paddle/fluid/platform/cuda_device_function.h

@@ -63,6 +63,7 @@ __device__ T reduceSum(T val, int tid, int len) {
     val += platform::CudaShuffleDownSync(mask, val, offset);
 
   if (tid < warpSize) shm[tid] = 0;
+  __syncthreads();
 
   if (tid % warpSize == 0) {
     shm[tid / warpSize] = val;