[add] benchmark scripts

Context

This MR is part of aidge_core#262 to add a benchmark system.

Major modifications

• add: generate_singleop_onnx.py to create a single operator valid onnx file
• make '_load_onnx2graphview' deprecated and use 'convert_onnx_to_aidge' instead
• add: 'convert_aidge_to_onnx' function