[Tutorial] Update Hardmax operator tutorial

Context

Fix tutorial for adding an operator.

This is a preliminary step before a further merge request to add unit tests docs for onnx_import.

Actually implementation of operators have changed in version aidge_core/aidge_backend_cpu >= v0.3.0 in particular:

• registrar templates
• attributes

Modified files

Modified all necessary files to update to the new implementation.

Rerun and commited the notbook.

Ref to the resulting notebook in the branch: https://gitlab.eclipse.org/cguillon/aidge/-/blob/dev-add-operator-tuto/examples/tutorials/Hardmax_operator/operator_adding.ipynb

Detailed major modifications

none

TODO

none