docs: def get_roots(self):
          """
          Returns a list of roots of the graph.
  
          Examples
          --------
          >>> from pgmpy.base import DAG
          >>> graph = DAG([('A', 'B'), ('B', 'C'), ('B', 'D'), ('E', 'B')])
          >>> graph.get_roots()
          ['A', 'E']
          """
          return [node for node, in_degree in dict(self.in_degree()).items() if in_degree == 0]