Conv refine (#20644) (#20671)

* add condition judgement for performance improvement test=develop

* add condition judgement for performance improvement test=develop

* refine code style test=develop

paddle/fluid/operators/conv_cudnn_op.cu

@@ -540,6 +540,7 @@ class CUDNNConvGradOpKernel : public framework::OpKernel<T> {
             workspace_size);
       }
 
+      if (!is_sys_pad) {
         std::vector<int> starts(transformed_input_channel.dims().size(), 0);
         std::vector<int> axes(transformed_input_channel.dims().size(), 0);
 
@@ -558,6 +559,7 @@ class CUDNNConvGradOpKernel : public framework::OpKernel<T> {
               ctx, &transformed_input_grad, &transformed_input_grad_channel,
               starts, axes);
         }
+      }
 
       if (channel_last) {
         TransToChannelLast<paddle::platform::CUDADeviceContext, T>(
@@ -982,6 +984,7 @@ class CUDNNConvDoubleGradOpKernel : public framework::OpKernel<T> {
             workspace_size);
       }
 
+      if (!is_sys_pad) {
         // reverse padded input
         std::vector<int> starts(X->dims().size(), 0);
         std::vector<int> axes(X->dims().size(), 0);
@@ -997,6 +1000,7 @@ class CUDNNConvDoubleGradOpKernel : public framework::OpKernel<T> {
           Slice_2<paddle::platform::CUDADeviceContext, T, 5>(
               ctx, &transformed_dX, &transformed_dX_channel, starts, axes);
         }
+      }
       if (channel_last) {
         TransToChannelLast<paddle::platform::CUDADeviceContext, T>(
             ctx, &transformed_dX_channel, dX);