code: def __next__(self):
      if not self._reduce_arg_blocks:
          raise StopIteration
  
      reduce_idx, reduce_arg_blocks = self._reduce_arg_blocks.pop(0)
      merge_idx = (
          self._stage.merge_schedule.get_merge_idx_for_reducer_idx(
              reduce_idx
          )
      )
      # Submit one partition of reduce tasks, one for each of the P
      # outputs produced by the corresponding merge task.
      # We also add the merge task arguments so that the reduce task
      # is colocated with its inputs.
      block, meta = self._shuffle_reduce.options(
          **self._ray_remote_args,
          **self._stage.get_merge_task_options(merge_idx),
          num_returns=2,
      ).remote(
          *self._reduce_args, *reduce_arg_blocks, partial_reduce=False
      )
      self._reduce_results.append((reduce_idx, block))
      return meta
  
imports: import logging
  import math
  from typing import Any, Callable, Dict, List, Optional, Tuple, TypeVar, Union
  import ray
  from ray.data._internal.block_list import BlockList
  from ray.data._internal.execution.interfaces import TaskContext
  from ray.data._internal.progress_bar import ProgressBar
  from ray.data._internal.remote_fn import cached_remote_fn
  from ray.data._internal.shuffle import ShuffleOp
  from ray.data.block import Block, BlockAccessor, BlockExecStats, BlockMetadata
  from ray.data.context import DataContext
  from ray.types import ObjectRef
  from ray.util.scheduling_strategies import NodeAffinitySchedulingStrategy
label_desc: Use image node_id 3 for calling the _ReduceStageIterator obj's underlying member method code with example usage: obj.__next__() and returns: meta