Stop the underlying :class:`SparkContext`.

def stop(self):
        """Stop the underlying :class:`SparkContext`.
        """
        self._sc.stop()
        # We should clean the default session up. See SPARK-23228.
        self._jvm.SparkSession.clearDefaultSession()
        self._jvm.SparkSession.clearActiveSession()
        SparkSession._instantiatedSession = None
        SparkSession._activeSession = None