docs: def get_factor_nodes(self):
          """
          Returns factors nodes present in the graph.
  
          Before calling this method make sure that all the factors are added
          properly.
  
          Examples
          --------
          >>> from pgmpy.models import FactorGraph
          >>> from pgmpy.factors.discrete import DiscreteFactor
          >>> G = FactorGraph()
          >>> G.add_nodes_from(['a', 'b', 'c'])
          >>> phi1 = DiscreteFactor(['a', 'b'], [2, 2], np.random.rand(4))
          >>> phi2 = DiscreteFactor(['b', 'c'], [2, 2], np.random.rand(4))
          >>> G.add_nodes_from([phi1, phi2])
          >>> G.add_factors(phi1, phi2)
          >>> G.add_edges_from([('a', phi1), ('b', phi1),
          ...                   ('b', phi2), ('c', phi2)])
          >>> G.get_factor_nodes()
          [<DiscreteFactor representing phi(b:2, c:2) at 0x4b8c7f0>,
           <DiscreteFactor representing phi(a:2, b:2) at 0x4b8c5b0>]
          """
          self.check_model()
  
          variable_nodes = self.get_variable_nodes()
          factor_nodes = set(self.nodes()) - set(variable_nodes)
          return list(factor_nodes)