取前n大元素，等同于 :func:`bigflow.transforms.max_elements(self, n, key) ` Args: n (int): 必须大于0 key (function, optional): 用于提取key的函数，与Python内置``max()``中的 ``key`` 参数相同 **options: 可配置选项 Returns:
(self, n, key=None, **options)
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