message: Improve tf.nest.is_nested docstring
diff: @@ -274,6 +274,37 @@ is_sequence_or_composite = _pywrap_utils.IsSequenceOrComposite
  def is_nested(seq):
  """Returns true if its input is a collections.abc.Sequence (except strings).
  + >>> tf.nest.is_nested("1234")
  + False
  +
  + >>> tf.nest.is_nested([1, 3, [4, 5]])
  + True
  +
  + >>> tf.nest.is_nested(((7, 8), (5, 6)))
  + True
  +
  + >>> tf.nest.is_nested([])
  + True
  +
  + >>> tf.nest.is_nested({"a": 1, "b": 2})
  + True
  +
  + >>> tf.nest.is_nested({"a": 1, "b": 2}.keys())
  + True
  +
  + >>> tf.nest.is_nested({"a": 1, "b": 2}.values())
  + True
  +
  + >>> tf.nest.is_nested({"a": 1, "b": 2}.items())
  + True
  +
  + >>> tf.nest.is_nested(set([1, 2]))
  + False
  +
  + >>> ones = tf.ones([2, 3])
  + >>> tf.nest.is_nested(ones)
  + False
  +
  Args:
  seq: an input sequence.
  