Fix sum_op's GetExpectedKernelType (#14112)

* fix sum_op's GetExpectedKernelType
test=develop

* fix ci fail
test=develop

paddle/fluid/framework/operator.cc

@@ -358,7 +358,7 @@ static bool VarIsTensor(const Variable* var) {
   return var->IsType<LoDTensor>() || var->IsType<SelectedRows>();
 }
 
-static const Tensor* GetTensorFromVar(Variable* var) {
+const Tensor* GetTensorFromVar(Variable* var) {
   if (var->IsType<LoDTensor>()) {
     return var->GetMutable<LoDTensor>();
   } else if (var->IsType<SelectedRows>()) {

paddle/fluid/framework/operator.h

@@ -63,6 +63,7 @@ inline std::string GradVarName(const std::string& var_name) {
 }
 
 proto::VarType::Type GetDataTypeOfVar(const Variable* var);
+const Tensor* GetTensorFromVar(Variable* var);
 
 class OperatorBase;
 class ExecutionContext;

paddle/fluid/operators/sum_op.cc

@@ -82,14 +82,16 @@ class SumOp : public framework::OperatorWithKernel {
     if (x_vars[0]->IsType<framework::LoDTensor>()) {
       int dtype = -1;
       for (auto& x_var : x_vars) {
-        auto& lod_tensor = x_var->Get<framework::LoDTensor>();
-        if (lod_tensor.numel() == 0) {
+        // FIXME(zcd): The input x_var may be SelectedRows or LoDTensor.
+        auto tensor = framework::GetTensorFromVar(
+            const_cast<framework::Variable*>(x_var));
+        if (tensor->numel() == 0) {
           continue;
         }
         if (dtype == -1) {
-          dtype = framework::ToDataType(lod_tensor.type());
+          dtype = framework::ToDataType(tensor->type());
         } else {
-          PADDLE_ENFORCE_EQ(dtype, framework::ToDataType(lod_tensor.type()));
+          PADDLE_ENFORCE_EQ(dtype, framework::ToDataType(tensor->type()));
         }
       }
       PADDLE_ENFORCE_NE(dtype, -1,