Create a layer stack based on the hyperparameter values.

def layer_stack_from_hparams(hparams, prefix):
  """Create a layer stack based on the hyperparameter values."""
  layers = hparams.get(prefix + "layers")
  return transformer.LayerStack(
      [layers_registry[l](hparams, prefix) for l in layers],
      dropout_rate=hparams.layer_prepostprocess_dropout,
      norm_epsilon=hparams.norm_epsilon)