content: from modeling.backbone import resnet, xception, drn, mobilenet
  
  def build_backbone(backbone, output_stride, BatchNorm):
      if backbone == 'resnet':
          return resnet.ResNet101(output_stride, BatchNorm)
      elif backbone == 'xception':
          return xception.AlignedXception(output_stride, BatchNorm)
      elif backbone == 'drn':
          return drn.drn_d_54(BatchNorm)
      elif backbone == 'mobilenet':
          return mobilenet.MobileNetV2(output_stride, BatchNorm)
      else:
          raise NotImplementedError
  