message: Update the docstring about validation_data.
  Make it more explicit about which types are not supported.
diff: @@ -573,6 +573,8 @@ class Model(network.Network, version_utils.ModelVersionSelector):
  For the first two cases, `batch_size` must be provided.
  For the last case, `validation_steps` could be provided.
  + Note that `validation_data` does not support all the data types that
  + are supported in `x`, eg, dict, generator or `keras.utils.Sequence`.
  shuffle: Boolean (whether to shuffle the training data
  before each epoch) or str (for 'batch'). This argument is ignored
  when `x` is a generator. 'batch' is a special option for dealing
  