fix unit test

paddle/fluid/operators/math/selected_rows_functor.cu

@@ -267,10 +267,15 @@ struct MergeAdd<platform::CUDADeviceContext, T> {
   void operator()(const platform::CUDADeviceContext& context,
                   const framework::SelectedRows& input,
                   framework::SelectedRows* output) {
-    framework::SelectedRows& out = *output;
     framework::Vector<int64_t> input_rows(input.rows());
+    if (input_rows.size() == 0) {
+      return;
+    }
+
+    framework::SelectedRows& out = *output;
     std::set<int64_t> row_set(input_rows.begin(), input_rows.end());
-    std::vector<int64_t> merge_rows(row_set.begin(), row_set.end());
+    std::vector<int64_t> merge_rows_cpu(row_set.begin(), row_set.end());
+    framework::Vector<int64_t> merge_rows(merge_rows_cpu);
 
     auto input_width = input.value().dims()[1];
 
@@ -313,8 +318,9 @@ struct MergeAdd<platform::CUDADeviceContext, T> {
                         "all input should have same height");
       merged_row_set.insert(input->rows().begin(), input->rows().end());
     }
-    std::vector<int64_t> merge_rows(merged_row_set.begin(),
+    std::vector<int64_t> merge_rows_cpu(merged_row_set.begin(),
                                     merged_row_set.end());
+    framework::Vector<int64_t> merge_rows(merge_rows_cpu);
 
     out.set_rows(merge_rows);
     out.set_height(input_height);
@@ -334,6 +340,9 @@ struct MergeAdd<platform::CUDADeviceContext, T> {
     for (auto* input : inputs) {
       auto* input_data = input->value().data<T>();
       auto& input_rows = input->rows();
+      if (input_rows.size() == 0) {
+        continue;
+      }
       dim3 grid1(input_rows.size(), 1);
 
       MergeAddKernel<T, 256><<<grid1, threads, 0, context.stream()>>>(