Use double to reduce

Author: reyoung

paddle/fluid/operators/layer_norm_op.cu

@@ -67,27 +67,27 @@ template <typename T, int BlockDim>
 __global__ void LayerNormForward(const T *x, const T *scale, const T *bias,
                                  T *y, T *mean, T *var, float epsilon,
                                  int feature_size) {
-  using BlockReduce = cub::BlockReduce<PairForLayerNorm<T>, BlockDim>;
+  using BlockReduce = cub::BlockReduce<PairForLayerNorm<double>, BlockDim>;
   __shared__ typename BlockReduce::TempStorage temp_storage;
 
   int beg_idx = blockIdx.x * feature_size + threadIdx.x;
   int end_idx = (blockIdx.x + 1) * feature_size;
 
   // Step 1: Reduce to calculate mean and var
-  T mean_val = static_cast<T>(0);
-  T var_val = static_cast<T>(0);
+  double mean_val = 0;
+  double var_val = 0;
   for (int i = beg_idx; i < end_idx; i += BlockDim) {
     T tmp = x[i];
     mean_val += tmp;
     var_val += (tmp * tmp);
   }
   auto pair = BlockReduce(temp_storage)
-                  .Reduce(PairForLayerNorm<T>(mean_val, var_val),
-                          PairForLayerNormAddFunctor<T>());
+                  .Reduce(PairForLayerNorm<double>(mean_val, var_val),
+                          PairForLayerNormAddFunctor<double>());
   if (threadIdx.x == 0) {
     auto tmp = pair.first_ / feature_size;
-    mean[blockIdx.x] = tmp;
-    var[blockIdx.x] = pair.second_ / feature_size - tmp * tmp;
+    mean[blockIdx.x] = static_cast<T>(tmp);
+    var[blockIdx.x] = static_cast<T>(pair.second_ / feature_size - tmp * tmp);
   }
   __syncthreads();
   mean_val = mean[blockIdx.x];