code: def test_flatten_list():
      """Tests flatten_list"""
      assert flatten_list([[]]) == []
      assert flatten_list([[0, 1, 2, 3]]) == [0, 1, 2, 3]
      assert flatten_list([[0], [1], [2], [3]]) == [0, 1, 2, 3]
      assert flatten_list([[0, 1], [2, 3]]) == [0, 1, 2, 3]
  
imports: import warnings
  from dataclasses import dataclass
  from typing import List
  import numpy
  import pandas
  import pytest
  import scipy
  from greykite.common import constants
  from greykite.common.evaluation import ElementwiseEvaluationMetricEnum
  from greykite.common.python_utils import apply_func_to_columns
  from greykite.common.python_utils import assert_equal
  from greykite.common.python_utils import dictionaries_values_to_lists
  from greykite.common.python_utils import dictionary_values_to_lists
  from greykite.common.python_utils import flatten_list
  from greykite.common.python_utils import get_integer
  from greykite.common.python_utils import get_pattern_cols
  from greykite.common.python_utils import group_strs_with_regex_patterns
  from greykite.common.python_utils import ignore_warnings
  from greykite.common.python_utils import mutable_field
  from greykite.common.python_utils import reorder_columns
  from greykite.common.python_utils import split_offset_str
  from greykite.common.python_utils import unique_dict_in_list
  from greykite.common.python_utils import unique_elements_in_list
  from greykite.common.python_utils import unique_in_list
  from greykite.common.python_utils import update_dictionaries
  from greykite.common.python_utils import update_dictionary
label_desc: Use image node_id 11 for calling a global function with example usage: test_flatten_list() without return types