Fix LSTM ONNX compatibility

Minor fixes related to #167.

• Add forcedInputsCategory attribute to MetaOperator, which allows to change the input category of the meta op inputs (which should not necessarily inherits from its inner nodes). Example: the meta op input should be a Param category but this input goes to a Squeeze operator inside the meta op. If inherited, the input category would be Data instead of Param.
• Added binding for set_upper_node.
• Fixed bug in GraphView::save().
• Changed behavior of Scheduler to initialize the consumer list to the output nodes.

python_binding/operator/pybind_MetaOperatorDefs.cpp

@@ -198,7 +198,8 @@ void init_MetaOperatorDefs(py::module &m) {
   .def(py::init<const char *, const std::shared_ptr<GraphView>&>(),
           py::arg("type"),
           py::arg("graph"))
-  .def("get_micro_graph", &MetaOperator_Op::getMicroGraph);
+  .def("get_micro_graph", &MetaOperator_Op::getMicroGraph)
+  .def("set_upper_node", &MetaOperator_Op::setUpperNode);
 
   m.def("meta_operator", &MetaOperator,
     py::arg("type"),