message: doctest fix ubuntu cpu test
  fix expected return for doctests with grayscale_to_rgb, rgb_to_grayscale
diff: @@ -1871,7 +1871,7 @@ def rgb_to_grayscale(images, name=None):
  >>> original = tf.constant([[[1.0, 2.0, 3.0]]])
  >>> converted = tf.image.rgb_to_grayscale(original)
  >>> print(converted.numpy())
  - array([[[1.8149]]], dtype=float32)
  + [[[1.8149]]]
  ```
  Args:
  @@ -1908,9 +1908,9 @@ def grayscale_to_rgb(images, name=None):
  >>> original = tf.constant([[[1.0], [2.0], [3.0]]])
  >>> converted = tf.image.grayscale_to_rgb(original)
  >>> print(converted.numpy())
  - array([[[1., 1., 1.],
  + [[[1., 1., 1.],
  [2., 2., 2.],
  - [3., 3., 3.]]], dtype=float32)
  + [3., 3., 3.]]]
  ```
  Args:
  