code: def test_get_constant_bn(self):
      self.network.add_edges_from(
          [
              (("D", 0), ("G", 0)),
              (("I", 0), ("G", 0)),
              (("D", 0), ("D", 1)),
              (("I", 0), ("I", 1)),
              (("D", 1), ("G", 1)),
              (("I", 1), ("G", 1)),
          ]
      )
      self.network.add_cpds(
          self.grade_cpd,
          self.d_i_cpd,
          self.diff_cpd,
          self.intel_cpd,
          self.i_i_cpd,
          self.grade_1_cpd,
      )
  
      bn = self.network.get_constant_bn(t_slice=0)
      self.assertEqual(
          set(bn.nodes()), {"D_0", "I_0", "G_0", "D_1", "I_1", "G_1"}
      )
      self.assertEqual(
          set(bn.edges()),
          {
              ("D_0", "G_0"),
              ("I_0", "G_0"),
              ("D_0", "D_1"),
              ("I_0", "I_1"),
              ("D_1", "G_1"),
              ("I_1", "G_1"),
          },
      )
      self.assertTrue(bn.check_model())
  
      bn = self.network.get_constant_bn(t_slice=1)
      self.assertEqual(
          set(bn.nodes()), {"D_1", "I_1", "G_1", "D_2", "I_2", "G_2"}
      )
      self.assertEqual(
          set(bn.edges()),
          {
              ("D_1", "G_1"),
              ("I_1", "G_1"),
              ("D_1", "D_2"),
              ("I_1", "I_2"),
              ("D_2", "G_2"),
              ("I_2", "G_2"),
          },
      )
      self.assertTrue(bn.check_model())
  
      bn = self.network.get_constant_bn(t_slice=2)
      self.assertEqual(
          set(bn.nodes()), {"D_2", "I_2", "G_2", "D_3", "I_3", "G_3"}
      )
      self.assertEqual(
          set(bn.edges()),
          {
              ("D_2", "G_2"),
              ("I_2", "G_2"),
              ("D_2", "D_3"),
              ("I_2", "I_3"),
              ("D_3", "G_3"),
              ("I_3", "G_3"),
          },
      )
      self.assertTrue(bn.check_model())
  
imports: import unittest
  import numpy
  import numpy.testing
  import pandas
  import pgmpy.tests.help_functions
  from pgmpy.factors.discrete import TabularCPD
  from pgmpy.inference import CausalInference, DBNInference, VariableElimination
  from pgmpy.models import BayesianNetwork
  from pgmpy.models import DynamicBayesianNetwork
label_desc: Use image node_id 2 for calling the TestDynamicBayesianNetworkMethods obj's underlying member method code with example usage: obj.test_get_constant_bn() without return types