fix: onnx imports

Context

This MR is intended to fix the import for the following operators:

• ReduceMean: add default value for axes
• ReduceSum: add default value for axes
• Squeeze: fill axes attribute from input if it is available
• Unsqueeze: fill axes attribute from input if it is available and return None if no axes are provided
• Transpose: support operator without perm attribute

This fixes issue #40

Modified files

• aidge_onnx/node_import/onnx_converters/reducemean.py, fix ReduceMean import;
• aidge_onnx/node_import/onnx_converters/reducesum.py, fix ReduceSum import;
• aidge_onnx/node_import/onnx_converters/squeeze.py, fix Squeeze import;
• aidge_onnx/node_import/onnx_converters/unsqueeze.py, fix Unsqueeze import;
• aidge_onnx/node_import/onnx_converters/transpose.py, fix Transpose import;

TODO

• Fix ReduceMean
• Fix ReduceSum
• Fix Squeeze
• Fix Unsqueeze
• Fix Transpose