sum selected rows check empty

paddle/fluid/operators/sum_op.h

@@ -116,8 +116,22 @@ class SumKernel : public framework::OpKernel<T> {
       auto *out = context.Output<SelectedRows>("Out");
       out->mutable_rows()->clear();
 
+      bool has_data = false;
+      for (auto &in : inputs) {
+        if (in->rows().size() > 0) {
+          has_data = true;
+          break;
+        }
+      }
+      if (has_data) {
         math::scatter::MergeAdd<DeviceContext, T> merge_add;
-      merge_add(context.template device_context<DeviceContext>(), inputs, out);
+        merge_add(context.template device_context<DeviceContext>(), inputs,
+                  out);
+      } else {
+        // no data, just set a empty out tensor.
+        out->mutable_value()->mutable_data<T>(framework::make_ddim({0}),
+                                              context.GetPlace());
+      }
     } else if (out_var->IsType<framework::LoDTensorArray>()) {
       auto &out_array = *out_var->GetMutable<framework::LoDTensorArray>();
       for (size_t i = in_place ? 1 : 0; i < in_vars.size(); ++i) {