[core] Bad MetaOp attributes

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What commit version of aidge do you use

• aidge_core: 0.3.1

Problem description

Attributes from elements of a MetaOperator cannot be accessed in python.

Reproducible example code

>>> import aidge_core as ai
>>> import aidge_backend_cpu
>>> 
>>> op = ai.PaddedConv2DOp([3,3], padding_dims=[1,1,1,1])
>>> op.attr
AttrDict({'Conv2D': {'dilation_dims': '???', 'kernel_dims': '???', 'stride_dims': '???'}, 'Conv2D#0': {'dilation_dims': '???', 'kernel_dims': '???', 'stride_dims': '???'}, 'Pad2D': {'begin_end_borders': '???', 'border_type': '???', 'border_value': '???'}, 'Pad2D#0': {'begin_end_borders': '???', 'border_type': '???', 'border_value': '???'}})
>>> op.attr.Conv2D
AttrDict({'dilation_dims': '???', 'kernel_dims': '???', 'stride_dims': '???'})
>>> op.attr.Conv2D.dilation_dims
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: map::at
>>> 
>>> op.attr.Conv2D#0
AttrDict({'dilation_dims': '???', 'kernel_dims': '???', 'stride_dims': '???'})
>>> op.attr.Conv2D#0.dilation_dims
AttrDict({'dilation_dims': '???', 'kernel_dims': '???', 'stride_dims': '???'})