message: call will call build
  Make the requirement clear. We don't have to build before __call__.
diff: @@ -924,6 +924,7 @@ class Layer(module.Module, version_utils.LayerVersionSelector):
  for `inputs` by the previous layer (if `input` did come from
  a layer that generated a corresponding mask, i.e. if it came from
  a Keras layer with masking support.
  + - If the layer is not built, the method will call `build`.
  Raises:
  ValueError: if the layer's `call` method returns None (an invalid value).
  