message: Add missing `watch` call to GradientTape documentation.
diff: @@ -669,6 +669,7 @@ class GradientTape(object):
  ```python
  x = tf.constant(3.0)
  with tf.GradientTape() as g:
  + g.watch(x)
  with tf.GradientTape() as gg:
  gg.watch(x)
  y = x * x
  