Dont let backend, etc. go out of scope (#3245)

test/backend_performance.cpp

@@ -98,7 +98,7 @@ TEST(benchmark, concat_32x1x200_axis1_6)
         auto result_tv = backend->create_tensor(element::f32, result_shape);
         result_tvs.push_back(result_tv);
 
-        std::function<void()> cb = [&]() {
+        std::function<void()> cb = [=]() {
             auto handle = backend->compile(f);
             handle->call_with_validate({result_tv}, input_vals);
         };