Fix split

Context

Small fix to load Split node when splits input is dynamic.
Instead of considering attributes to set output number, with this MR we take the output number from the graph node instead of the attributes and inputs. Also a small fix, when converting aidge name to onnx name, a small fix is added when node name is empty.

Modified files

• split.py, nb_outputs is computed using the graph and not the attribs;
• utils.py, fix onnx_to_aidge_name when node name is empty;

TODO

• Fix split import
• Fix onnx_to_aidge_name