对多个输入PCollection求笛卡尔积，返回一个PCollection Args: *pcollections: 输入PCollection **options: 可配置选项 Returns: PCollection: 笛卡尔积 >>> from bigflow import transforms >>> p1 = _pipeline.parallelize([1, 2, 3]) >>> p2 = _pipeline.parallelize(['a', 'b', 'c']) >>> tr
(*pcollections, **options)
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