input: Write a python 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([[10, 20], [30, 40, 50], [60]]) == [10, 20, 30, 40, 50, 60]
  assert flatten_list([['a', 'b'], ['c', 'd', 'e'], ['f']]) == ['a', 'b', 'c', 'd', 'e', 'f']
output: def flatten_list(nested_list):
      return [element for sublist in nested_list for element in sublist]