Also better warning message for conv operator constructor

- split the function in two for more comprehesive structure
- change name from 'removeConstantOFShape' to 'foldConstantOfShape' since it describe way better what the function does
- enhance function structure, Producer has got only one child Node by construction. No need to check

Move constructor, and clone functions from operators' header to src and add src file for PaddedConv and PaddedConvDepthWise

Move constructor, and clone functions from operators' header to src and add src file for PaddedConv and PaddedConvDepthWise

- [Add] attribute() member function of Oeprator. Return nullptr if no attributes
- [Fix] Scaling attributes names from camelCase to PascalCase

- Move code definition in an external cpp file in src directory
- Remove InChannels, OutChannels, NbChannels from the list of attributes as they are only used for creating the associated Producer. They are kept as parameters of the Operator factory function to continue creating Weight and Bias Producers automatically. The number of in/out channels is now based on the Weight parameter
- 'Conv_Op::inChannels()', 'Conv_Op::outChannels()', 'ConvDepthWise::nbChannels()' functons added

- Move code definition in an external cpp file in src directory
- Remove InChannels, OutChannels, NbChannels from the list of attributes as they are only used for creating the associated Producer. They are kept as parameters of the Operator factory function to continue creating Weight and Bias Producers automatically. The number of in/out channels is now based on the Weight parameter
- 'Conv_Op::inChannels()', 'Conv_Op::outChannels()', 'ConvDepthWise::nbChannels()' functons added