class_docstr: Base class inherited by all feed-forward network classes.
  
  Args:
      hparams (dict, optional): Hyperparameters. Missing
          hyperparamerter will be set to default values. See
          :meth:`default_hparams` for the hyperparameter sturcture and
          default values.
  
  See :meth:`_build` for the inputs and outputs.
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docstr: Appends a layer to the end of the network. The method is only
  feasible before :attr:`_build` is called.
  
  Args:
      layer: A :tf_main:`tf.layers.Layer <layers/Layer>` instance, or
          a dict of layer hyperparameters.
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code: def append_layer(self, layer):
      """Appends a layer to the end of the network. The method is only
      feasible before :attr:`_build` is called.
  
      Args:
          layer: A :tf_main:`tf.layers.Layer <layers/Layer>` instance, or
              a dict of layer hyperparameters.
      """
      if self._built:
          raise TexarError(
              "`FeedForwardNetwork.append_layer` can be "
              "called only before `_build` is called."
          )
  
      with tf.variable_scope(self.variable_scope):
          layer_ = layer
          if not isinstance(layer_, tf.layers.Layer):
              layer_ = get_layer(hparams=layer_)
          self._layers.append(layer_)
          layer_name = uniquify_str(layer_.name, self._layer_names)
          self._layer_names.append(layer_name)
          self._layers_by_name[layer_name] = layer_
  
imports: import tensorflow
  from texar.tf.module_base import ModuleBase
  from texar.tf.utils import TexarError
  from texar.tf.core.layers import get_layer
  from texar.tf.utils.utils import uniquify_str
  from texar.tf.utils.mode import is_train_mode
label_desc: Use image node_id 4 for calling the FeedForwardNetworkBase obj's underlying member method code with example usage: obj.append_layer(layer) without return types