Function aggregate

将给定的PCollection按照初始值、初段聚合方法和汇总方法聚合为PObject 假设输入类型I，输出类型为O，则zero、aggregate_fn、combine_fn的期望签名为: zero: O或zero() => O aggregate_fn: aggregate_fn(O, I) => O (aggregate_fn的第一个参数允许被修改) combine_fn: combine_fn(O, O) => O (combine_fn的第一个参数允许被修改) 在执行时aggregate会把输入pcollectio

(pcollection, zero, aggregate_fn, combine_fn, *side_inputs, **options)