This operator takes as input potentially multiple input data quanta and outputs multiple input data quanta.
Since Rheem is not a physical execution engine, its notion of partitions is rather loose. Implementors
of this operator should guarantee that the partitions are distinct in their data quanta and that all partitions together
are complete w.r.t. the data quanta.
However, no further assumptions on partitions shall be made, such as: whether partitions can be iterated multiple
times; whether partitions can be empty; whether there is a partition on each machine on distributed platforms;
or whether partitions have a certain sorting order.