fix invalid read of pnorm gradient function

fix invalid read of pnorm gradient function and delete the unused code

paddle/fluid/operators/p_norm_op.cu

@@ -99,39 +99,25 @@ __global__ void PnormGradient(const T* x, const T* x_norm, const T* y_grad,
                               const float porder, const int pre,
                               const int axis_n, const int post, const T eps,
                               T* x_grad) {
-  typedef cub::BlockReduce<T, BlockDim> BlockReduce;
-  __shared__ typename BlockReduce::TempStorage temp_storage_sum;
   // dx = (x/pnorm_broadcast).pow(p-1) * norm_dy.broadcast * sign(x)
   int num = pre * post;
   auto porder_grad = static_cast<T>(porder - 1.0f);
   for (int i = blockIdx.x; i < num; i += gridDim.x) {
-    T sum = 0.0;
-    __shared__ T row_sum;
-    __shared__ T row_sqrt_norm;
-    __shared__ T row_norm;
+    __shared__ T pnorm_i;
+    __shared__ T yout_i;
 
     auto base = (i / post) * post * axis_n + (i % post);
 
-    for (int j = threadIdx.x; j < axis_n; j += blockDim.x) {
-      int index = base + j * post;
-      sum += x[index] * y_grad[index];
-    }
-    T reduce_result = BlockReduce(temp_storage_sum).Sum(sum);
-
     if (threadIdx.x == 0) {
-      row_sum = reduce_result;
-      row_sqrt_norm = x_norm[i];
-      row_norm = row_sqrt_norm * row_sqrt_norm;
+      pnorm_i = x_norm[i];
+      yout_i = y_grad[i];
     }
-    __syncthreads();
 
-    const T pnorm_i = x_norm[i];
-    const T yout_i = y_grad[i];
+    __syncthreads();
 
     for (int j = threadIdx.x; j < axis_n; j += blockDim.x) {
       int index = base + j * post;
       const T x_ij = inline_abs(x[index]);
-      const T dy_ij = y_grad[index];
       x_grad[index] = inline_pow(x_ij, porder_grad) /
                       (inline_pow(pnorm_i, porder_grad) + eps) * yout_i *
                       inline_sign(x[index]);