fix pool_op bug (#7879)

paddle/operators/pool_op.h

@@ -139,10 +139,8 @@ class PoolGradKernel : public framework::OpKernel<T> {
     auto& dev_ctx = context.template device_context<DeviceContext>();
     if (in_x_grad) {
       in_x_grad->mutable_data<T>(context.GetPlace());
-      auto temp = framework::EigenVector<T>::Flatten(*in_x_grad);
-      temp.device(
-          *context.template device_context<DeviceContext>().eigen_device()) =
-          temp.constant(static_cast<T>(0));
+      paddle::operators::math::SetConstant<DeviceContext, T> set_constant;
+      set_constant(dev_ctx, in_x_grad, 0.0);
 
       switch (ksize.size()) {
         case 2: {