Cannot export some imported onnx models

What commit version of aidge do you use

• aidge_core: dev
• aidge_onnx: dev

Problem description

When I try to export some models, that were first imported from ONNX, I get an empty onnx file, but not with all models. Also just using aidge_core.sequential and exporting works fine.

What I'm using:

aidge_model = aidge_onnx.load_onnx("model_mnist_fc.onnx", verbose=False)
aidge_core.remove_flatten(aidge_model) 

aidge_model.save("saved_model") # this works fine

input_node = aidge_core.Producer([1, 1, 28, 28], "XXX")
input_node.add_child(aidge_model)
aidge_model.add(input_node)

# Set up the backend
aidge_model.set_datatype(aidge_core.dtype.float32)
aidge_model.set_backend("cpu")

aidge_onnx.export_onnx(aidge_model, "model_normal.onnx")

I cannot really see a common denominator in what fails and what does not fails so:

Model working: model_mnist_fc.onnx (28, 28, 1)

Model not working: ConvNet.onnx (from PTQ tuto (28, 28, 1)) or model_matmul.onnx (2,)