Added possibility to create a GenericOp from any Operator

Added possibility to create a GenericOp from any Operator.

One envisionned use-case is to be able to replace actual operators with GenericOp that would behave differently even regarding the size of inputs/outputs tensors. For example, a Conv that works on 4-bits packed data, which input/output size would be different from the standard Conv operator because of memory alignments. This would be useful for the export of sub 8-bits kernels in the Cortex-M export.