使用给定的key对元素排序，等同于 :func:`bigflow.transforms.sort_by(self, fn, reverse) ` Args: key (function, optional): 用于提取key的函数，与Python内置``sort()``中的 ``key`` 参数相同 reverse (bool): 若True则降序排列，否则为升序排列 Returns: PC
(self, key, reverse=False)
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