code: def test_flatten(ray_start_regular_shared):
      it = from_items([[1, 2], [3, 4]], 1).flatten()
      assert (
          repr(it)
          == "ParallelIterator[from_items[list, 2, shards=1].flatten()]"
      )
      assert list(it.gather_sync()) == [1, 2, 3, 4]
  
imports: import sys
  import time
  import collections
  from collections import Counter
  import pytest
  import ray
  from ray.util.iter import from_items, from_iterators, from_range, from_actors, ParallelIteratorWorker, LocalIterator
  from ray._private.test_utils import Semaphore
label_desc: Use image node_id 24 for calling a global function with example usage: test_flatten(ray_start_regular_shared) without return types