将给定的PCollection按照一个初始值和方法聚合为PObject 假设输入类型为I，输出类型为O，则zero、accumulate_fn的期望签名为: zero: O或zero() => O accumulate_fn: accumulate_fn(O, I) => O (accumulate_fn的第一个参数允许被修改) Args: pcollection (PCollection): 输入PCollection zero (value or function): 初始值，或是一个返回初始值的方法
(pcollection, zero, accumulate_fn, *side_inputs, **options)
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