sum_op selectedRows dim bug fix

paddle/fluid/operators/sum_op.h

@@ -105,8 +105,15 @@ class SumKernel : public framework::OpKernel<T> {
         auto &sel_row = get_selected_row(i);
         first_dim += sel_row.rows().size();
       }
-      auto in_dim =
-          framework::vectorize(get_selected_row(N - 1).value().dims());
+
+      std::vector<int64_t> in_dim;
+      for (int i = 0; i < N; i++) {
+        auto &sel_row = get_selected_row(i);
+        if (sel_row.rows().size() > 0) {
+          in_dim = framework::vectorize(sel_row.value().dims());
+          break;
+        }
+      }
       in_dim[0] = static_cast<int64_t>(first_dim);
 
       out_value->Resize(framework::make_ddim(in_dim));