Update conv and convdepth wise operators

- 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