Unecessary include of backend cpu

https://gitlab.eclipse.org/eclipse/aidge/aidge_export_arm_cortexm/-/blob/main/aidge_export_arm_cortexm/memory.py?ref_type=heads#L2

We shouldn't include backend cpu here !

Note: same here https://gitlab.eclipse.org/eclipse/aidge/aidge_export_cpp/-/blob/main/aidge_export_cpp/memory.py?ref_type=heads#L2

By the way, memory.py should be added to aidge_core instead of being duplicated !

added Enhancement label

mentioned in issue aidge_core#120 (closed)

Fixed in :!11 (merged)

closed