Add support for multiple Network

Add support for the following models:

• MLP
• DSCNN
• MobileNetv1
• ResNet18

This means the added support of the following operators (Float32/NHWC):

• ReLU
• Conv2D
• PadConv2D
• ConvDepthWise2D
• PadConvDepthWise2D
• AveragePooling2D
• GlobalAveragePooling2D
• SoftMax
• Elemwise
  • Add
  • Sub
  • Mul
  • Div

Furthermore, I integrated the ONNX simplify function in order to simplif graph before export.

Note

ONNX simplify recipes list should be moved to Aidge see aidge#1210

The following scirpt was used to test the export: xnn_test