fix optional inputs in node treated as input for the graph (Fix optional bias in Conv operator)

Context

Conv operators may have optional Bias. In this case, the missing inputs are treated as free available input of the GraphView.

Modified files

• modified: include/aidge/graph/Node.hpp
• modified: src/graph/Node.cpp

Detailed major modifications

• getNbFreeDataInputs : return only the number of available Data and DataOptional (not Params)
• getFirstFreeDataInput : return the first non assignated Data or DataOptional
• AddChild : Instead of getting the first node from the GraphView inputnode, it finds the first input node with data available (which should be unique)