Rework Gather, Slice and Reshape

For the operators Gather, Slice and Reshape, a previous work was done to make inputs into attributes, this issue is to fix that by allowing initializing these operator either by inputs or by attributes.

Reshape: MR_Core , MR_ONNX
Gather and Slice: MR_Core , MR_ONNX

Should partly fix aidge#54 (closed) and https://gitlab.eclipse.org/eclipse/aidge/aidge/-/issues/70

assigned to @hrouis

changed the description

added StatusWork in Progress label

added PriorityHigh label

Every related MR was merged. Closing.

closed