Constant producer are trained

What commit version of aidge do you use

• aidge_core: dev
• aidge_learning: dev

Problem description

Constant producers are trained in the learning tutorial. In the tutorial trainable parameters are fetched using aidge_core.producers(model) which outputs all producers of the model even if they are created with constant = True. Thus when we use opt.update they are also updated even if they should be constant.

Either the update should check if the producer is constant or we should not pass it as a parameter. As we pass tensors directly to the optimizer the second option might be easier and we just add a helper like non_constant_producers in GraphViewHelper.cpp.