A partition grouper that generates partition groups given the list of topic-partitions.
This grouper also acts as the stream task creation function along with partition distribution
such that each generated partition group is assigned with a distinct TaskId;
the created task ids will then be assigned to Kafka Streams instances that host the stream
Returns a map of task ids to groups of partitions. A partition group forms a task, thus, partitions that are
expected to be processed together must be in the same group.
Note that the grouping of partitions need to be sticky such that for a given partition, its assigned
task should always be the same regardless of the input parameters to this function. This is to ensure task's
local state stores remain valid through workload rebalances among Kafka Streams instances.
The default partition grouper implements this interface by assigning all partitions across different topics with the same
partition id into the same task. See DefaultPartitionGrouper for more information.