code: private long numDepthWiseParams(SeparableConvolution2D layerConf) {
          int[] kernel = layerConf.getKernelSize();
          val nIn = layerConf.getNIn();
          val depthMultiplier = layerConf.getDepthMultiplier();
  
          return nIn * depthMultiplier * kernel[0] * kernel[1];
      }
docs: For each input feature we separately compute depthMultiplier many
  output maps for the given kernel size
  
  @param layerConf layer configuration of the separable conv2d layer
  @return number of parameters of the channels-wise convolution operation