message: Add specification of an image batch for `tf.image.ssim`. Also fix wrong example.
diff: @@ -4252,9 +4252,14 @@ def ssim(img1,
  Example:
  ```python
  - # Read images from file.
  - im1 = tf.decode_png('path/to/im1.png')
  - im2 = tf.decode_png('path/to/im2.png')
  + # Read images (of size 255 x 255) from file.
  + im1 = tf.image.decode_image(tf.io.read_file('path/to/im1.png'))
  + im2 = tf.image.decode_image(tf.io.read_file('path/to/im2.png'))
  + tf.shape(im1) # `img1.png` has 3 channels; shape is `(255, 255, 3)`
  + tf.shape(im2) # `img2.png` has 3 channels; shape is `(255, 255, 3)`
  + # Add an outer batch for each image.
  + im1 = tf.expand_dims(im1, axis=0)
  + im2 = tf.expand_dims(im2, axis=0)
  # Compute SSIM over tf.uint8 Tensors.
  ssim1 = tf.image.ssim(im1, im2, max_val=255, filter_size=11,
  filter_sigma=1.5, k1=0.01, k2=0.03)
  @@ -4268,8 +4273,10 @@ def ssim(img1,
  ```
  Args:
  - img1: First image batch.
  - img2: Second image batch.
  + img1: First image batch. 4-D Tensor of shape `[batch, height, width,
  + channels]`.
  + img2: Second image batch. 4-D Tensor of shape `[batch, height, width,
  + channels]`.
  max_val: The dynamic range of the images (i.e., the difference between the
  maximum the and minimum allowed values).
  filter_size: Default value 11 (size of gaussian filter).
  