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.