code: def __init__(
      self,
      map_stage_iter: _MapStageIterator,
      shuffle_merge,
      stage: _PushBasedShuffleStage,
      reduce_args,
  ):
      self._map_stage_iter = map_stage_iter
      self._shuffle_merge = shuffle_merge
      self._stage = stage
      self._reduce_args = reduce_args
  
      self._merge_idx = 0
      self._map_result_buffer = None
      # Final outputs from the map-merge stage.
      # This is a map from merge task index to a nested list of merge results
      # (ObjectRefs). Each merge task index corresponds to a partition of P
      # final reduce tasks.
      self._all_merge_results = [
          []
          for _ in range(
              self._stage.merge_schedule.num_merge_tasks_per_round
          )
      ]
  
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 1 to create a new _MergeStageIterator object  with example: obj = _MergeStageIterator(map_stage_iter, shuffle_merge, stage, reduce_args)