CoGroupByKey returns a dictionary of {KeyType, List[ValueType]} but, as
GroupByKey, should return an Iterable.Change: https://github.com/apache/beam/pull/22984 Please look at this doc <https://docs.google.com/document/d/1RIzm8-g-0CyVsPb6yasjwokJQFoKHG4NjRUcKHKINu0> if you need more details. Feel free to comment.
