message: Add a warning message for usage of feeding keras.Input() to sequential model.
  The keras.Input() is designed for functional model, and might cause some use confusion when calling model.summary() or plot the model.
  See for more information.
diff: @@ -178,6 +178,10 @@ class Sequential(functional.Functional):
  origin_layer = layer._keras_history[0]
  if isinstance(origin_layer, input_layer.InputLayer):
  layer = origin_layer
  + logging.warning(
  + 'Please add `keras.layers.InputLayer` instead of `keras.Input` to '
  + 'Sequential model. `keras.Input` is intended to be used by '
  + 'Functional model.')
  if isinstance(layer, module.Module):
  if not isinstance(layer, base_layer.Layer):
  