取最小元素, 等同于 :func:`bigflow.transforms.min(self, key) ` Args: key (function, optional): 用于提取key的函数，与Python内置``min()``中的 ``key`` 参数相同 **options: 可配置选项 Returns: PObject: 最小元素 >>> p.parallelize([3
(self, key=None, **options)
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