message: Add a usage example for tf.is_tensor
diff: @@ -982,6 +982,16 @@ def is_tensor(x): # pylint: disable=invalid-name
  If `is_tensor(x)` returns `True`, it is safe to assume that `x` is a tensor or
  can be converted to a tensor using `ops.convert_to_tensor(x)`.
  + Usage example:
  + >>> x = tf.constant([[1,2,3],[4,5,6],[7,8,9]])
  + >>> print(x.shape)
  + (3, 3)
  + >>> print(tf.is_tensor(x))
  + True
  + >>> y = "Hello World"
  + >>> print(tf.is_tensor(y))
  + False
  +
  Args:
  x: A python object to check.
  