input: Write a python function to print all elements of a nested list with indentation indicating depth.
  assert print_nested_list([1, 2, [3, 4], [5, [6, 7, 8]]]) == "1\n2\n  3\n  4\n  5\n    6\n    7\n    8\n"
  assert print_nested_list(['a', ['b', 'c'], ['d', ['e', 'f']]]) == "a\n  b\n  c\n  d\n    e\n    f\n"
  assert print_nested_list([['apple', 'banana'], ['cherry', ['date', 'fig']], 'grape']) == "  apple\n  banana\n  cherry\n    date\n    fig\ngrape\n"
output: def print_nested_list(nested_list, depth=0):
      result = ''
      for item in nested_list:
          if isinstance(item, list):
              result += print_nested_list(item, depth + 1)
          else:
              result += '  ' * depth + str(item) + '\n'
      return result