add test for cxx and light api of getoutput. test=develop

lite/api/cxx_api_test.cc

@@ -129,6 +129,12 @@ TEST(CXXApi, load_model_naive) {
                              0.8938774});
 
   auto* output_tensor = predictor.GetOutput(0);
+  const std::vector<std::string> output_names = predictor.GetOutputNames();
+  for (int i = 0; i < outputs.size(); i++) {
+    LOG(INFO) << "output_names[" << i << "]:" << output_names[i];
+  }
+  auto* output_tensor_by_name = predictor.GetOutput(output_names[0]);
+  CHECK(output_tensor == output_tensor_by_name);
   auto output_shape = output_tensor->dims().Vectorize();
   ASSERT_EQ(output_shape.size(), 2);
   ASSERT_EQ(output_shape[0], 1);

lite/api/light_api_test.cc

@@ -51,6 +51,8 @@ TEST(LightAPI, load) {
   predictor.Run();
 
   const auto* output = predictor.GetOutput(0);
+  const auto* output_by_name = predictor.GetOutput(outputs[0]);
+  CHECK(output == output_by_name);
   const float* raw_output = output->data<float>();
 
   for (int i = 0; i < 10; i++) {