message: add usage examples
  added usage examples for tf.image.rgb_to_grayscale and tf.image.grayscale_to_rgb
diff: @@ -1874,6 +1874,13 @@ def rgb_to_grayscale(images, name=None):
  Returns:
  The converted grayscale image(s).
  +
  + Usage Example:
  + ```python
  + import tensorflow as tf
  + original = tf.random.normal(shape=(256, 256, 3))
  + converted = tf.image.rgb_to_grayscale(original)
  + ```
  """
  with ops.name_scope(name, 'rgb_to_grayscale', [images]) as name:
  images = ops.convert_to_tensor(images, name='images')
  @@ -1903,6 +1910,13 @@ def grayscale_to_rgb(images, name=None):
  Returns:
  The converted grayscale image(s).
  +
  + Usage Example:
  + ```python
  + import tensorflow as tf
  + original = tf.random.normal(shape=(256, 256, 1))
  + converted = tf.image.grayscale_to_rgb(original)
  + ```
  """
  with ops.name_scope(name, 'grayscale_to_rgb', [images]) as name:
  images = _AssertGrayscaleImage(images)
  