Constructs a ResNet-50 model.

def resnet50(num_classes=1000, pretrained='imagenet'):
    """Constructs a ResNet-50 model.
    """
    model = models.resnet50(pretrained=False)
    if pretrained is not None:
        settings = pretrained_settings['resnet50'][pretrained]
        model = load_pretrained(model, num_classes, settings)
    model = modify_resnets(model)
    return model