input: Write a function to flatten a nested list.
  assert flatten_list([[1, 2, 3], [4, 5, 6], [7, 8, 9]]) == [1, 2, 3, 4, 5, 6, 7, 8, 9]
  assert flatten_list([[1], [2, 3, 4], [5, 6], [7, 8, 9]]) == [1, 2, 3, 4, 5, 6, 7, 8, 9]
  assert flatten_list([[], [1, 2], [], [3], [4, 5, 6]]) == [1, 2, 3, 4, 5, 6]
output: def flatten_list(nested_list):
      return [item for sublist in nested_list for item in sublist]