[Feat](Exports) Utilitary function and a small fix for cpp export refactor

Context

This aidge_export_cpp MR brings a new way to group operators before the export step, which will hopefully be more easy to grasp.
In this context, two changes were needed in aidge_core :

Utilitary function

The get_node(node, <node_type>) function allows to easily browse a metaOp, returning the first node encountered with the given node type.
Future versions should handle the cases where several nodes of the same type are present in the given MetaOp.

Small fix

This new registering method is implemented such as the export nodes inherit from one another. For instance, the PadConv export node will inherit from the Conv export node. In this context, there was an issue with the @register operator which was returning the class through a wrapper. This has been solved removing this wrapper system, which does not seem to be useful elsewhere.