优化这段代码def xception(num_classes=1000, pretrained='imagenet', replace_stride_with_dilation=None): model = Xception(num_classes=num_classes, replace_stride_with_dilation=replace_stride_with_dilation) if pretrained: settings = pretrained_settings['xception'][pretrained] assert num_classes == settings['num_classes'], \ "num_classes should be {}, but is {}".format(settings['num_classes'], num_classes) model = Xception(num_classes=num_classes, replace_stride_with_dilation=replace_stride_with_dilation) model.load_state_dict(model_zoo.load_url(settings['url'])) # TODO: ugly model.last_linear = model.fc del model.fc return model

可以通过将模型定义和预训练权重加载分开，以更好地组织代码。例如，可以创建两个函数，一个用于定义模型，一个用于加载预训练权重。这样可以使代码更加清晰易懂。另外，也可以将 `TODO: ugly` 标记改为更具体的描述，以便后续改进。下面是一种可能的优化方案：

def xception(num_classes=1000, pretrained='imagenet', replace_stride_with_dilation=None):
    model = Xception(num_classes=num_classes, replace_stride_with_dilation=replace_stride_with_dilation)
    if pretrained:
        state_dict = load_pretrained_xception(pretrained)
        model.load_state_dict(state_dict)
        model.last_linear = model.fc
        del model.fc
    return model

def load_pretrained_xception(pretrained):
    settings = pretrained_settings['xception'][pretrained]
    num_classes = settings['num_classes']
    assert num_classes == 1000, "num_classes should be 1000, but is {}".format(num_classes)
    state_dict = model_zoo.load_url(settings['url'])
    return state_dict