class_docstr: GraphTopology for DAG models
    
code: def get_parents_placeholder(self):
      return self.parents_placeholder
  
imports: from __future__ import print_function
  from __future__ import division
  from __future__ import unicode_literals
  import warnings
  import numpy
  import tensorflow
  from deepchem.nn.copy import Input
  from deepchem.feat.mol_graphs import ConvMol
label_desc: Use image node_id 2 for calling the DAGGraphTopology obj's underlying member method code with example usage: obj.get_parents_placeholder() and returns: self