message: Add usage example to pad_to_bounding_box
diff: @@ -850,6 +850,34 @@ def pad_to_bounding_box(image, offset_height, offset_width, target_height,
  This op does nothing if `offset_*` is zero and the image already has size
  `target_height` by `target_width`.
  + Usage Example:
  + >>> x = [[[1.0, 2.0, 3.0],
  + ... [4.0, 5.0, 6.0]],
  + ... [[7.0, 8.0, 9.0],
  + ... [10.0, 11.0, 12.0]]]
  + >>> padded_image = tf.image.pad_to_bounding_box(x, 1, 1, 4, 4)
  + >>> print(padded_image)
  + <tf.Tensor: shape=(4, 4, 3), dtype=float32, numpy=
  + array([[[ 0., 0., 0.],
  + [ 0., 0., 0.],
  + [ 0., 0., 0.],
  + [ 0., 0., 0.]],
  +
  + [[ 0., 0., 0.],
  + [ 1., 2., 3.],
  + [ 4., 5., 6.],
  + [ 0., 0., 0.]],
  +
  + [[ 0., 0., 0.],
  + [ 7., 8., 9.],
  + [10., 11., 12.],
  + [ 0., 0., 0.]],
  +
  + [[ 0., 0., 0.],
  + [ 0., 0., 0.],
  + [ 0., 0., 0.],
  + [ 0., 0., 0.]]], dtype=float32)>
  +
  Args:
  image: 4-D Tensor of shape `[batch, height, width, channels]` or 3-D Tensor
  of shape `[height, width, channels]`.
  