This PR removes compiler warnings like this one:

```
src/ngraph/frontend/onnx_import/utils/reduction.hpp:73:28: warning: prior to the resolution
  of a defect report against ISO C++11, local variable 'op_node' would have been copied
  despite being returned by name, due to its not matching the function return type
  ('shared_ptr<ngraph::Node>' vs 'shared_ptr<ngraph::op::ArgMin>') [-Wreturn-std-move-in-c++11]
                  return op_node;
                         ^~~~~~~
```