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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def model_builds(self):
 """ Access the model_builds :returns: twilio.rest.autopilot.v1.assistant.model_build.ModelBuildList :rtype: twilio.rest.autopilot.v1.assistant.model_build.ModelBuildList """

        if self._model_builds is None:
            self._model_builds = ModelBuildList(self._version, assistant_sid=self._solution['sid'], )
        return self._model_builds