message: Fix: add GDN to __init__. Also put it in alphabetical order.
diff: @@ -32,6 +32,8 @@ See the @{$python/contrib.layers} guide.
  @@embedding_lookup_unique
  @@flatten
  @@fully_connected
  +@@GDN
  +@@gdn
  @@layer_norm
  @@linear
  @@max_pool2d
  