class_docstr: Base class inherited by all seq2seq model classes.
  
  .. document private functions
  .. automethod:: _build
class_docstr_tok: ["Base","class","inherited","by","all","seq2seq","model","classes",".","..","document","private","functions","..","automethod",":",":","_build"]
code: def _build_decoder(self):
      raise NotImplementedError
  
imports: import tensorflow
  from texar.tf.models.model_base import ModelBase
  from texar.tf.losses.mle_losses import sequence_sparse_softmax_cross_entropy
  from texar.tf.data.data.paired_text_data import PairedTextData
  from texar.tf.core.optimization import get_train_op
  from texar.tf.hyperparams import HParams
  from texar.tf.utils import utils
  from texar.tf.utils.variables import collect_trainable_variables
label_desc: Use image node_id 6 for calling the Seq2seqBase obj's underlying member method code with example usage: obj._build_decoder() without return types