class_docstr: Base class for a model description.
  
  It is used for the simple training interface described in
  `Training Interface Tutorial <https://tensorpack.readthedocs.io/tutorial/training-interface.html>`_.
  
  Subclass is expected to implement :meth:`inputs` and :meth:`build_graph`, as they
  together define a tower function.
docstr: A subclass is expected to implement this method.
  
  Build the whole symbolic graph.
  This is supposed to be part of the "tower function" when used with :class:`TowerTrainer`.
  
  Args:
      args ([tf.Tensor]): tensors that matches the list of inputs defined by ``inputs()``.
  
  Returns:
      In general it returns nothing, but a subclass
      may require it to return necessary information to build the trainer.
      For example, `SingleCostTrainer` expect this method to return the cost tensor.
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code: def build_graph(self, *args):
      """
      A subclass is expected to implement this method.
  
      Build the whole symbolic graph.
      This is supposed to be part of the "tower function" when used with :class:`TowerTrainer`.
  
      Args:
          args ([tf.Tensor]): tensors that matches the list of inputs defined by ``inputs()``.
  
      Returns:
          In general it returns nothing, but a subclass
          may require it to return necessary information to build the trainer.
          For example, `SingleCostTrainer` expect this method to return the cost tensor.
      """
      raise NotImplementedError()
  
file_path: tensorpack/tensorpack/train/model_desc.py
imports: import tensorflow
  from ..utils.argtools import memoized_method
  from ..tfutils.common import get_op_tensor_name
  from ..tfutils.tower import get_current_tower_context
  from ..compat import _backport_tensor_spec, tfv1
label_desc: Use image node_id 4 for calling the ModelDescBase obj's underlying member method code with example usage: obj.build_graph() without return types