def backbone(backbone_name):
    """ Returns a backbone object for the given backbone.
    """
    if 'densenet' in backbone_name:
        from .densenet import DenseNetBackbone as b
    elif 'seresnext' in backbone_name or 'seresnet' in backbone_name or 'senet' in backbone_name:
        from .senet import SeBackbone as b
    elif 'resnet' in backbone_name:
        from .resnet import ResNetBackbone as b
    elif 'mobilenet' in backbone_name:
        from .mobilenet import MobileNetBackbone as b
    elif 'vgg' in backbone_name:
        from .vgg import VGGBackbone as b
    elif 'EfficientNet' in backbone_name:
        from .effnet import EfficientNetBackbone as b
    else:
        raise NotImplementedError('Backbone class for  \'{}\' not implemented.'.format(backbone))

    return b(backbone_name)
 Returns a backbone object for the given backbone.
    
keras_retinanet/models/__init__.py