Helper function to update the _optimized_pipeline field.

def _update_top_pipeline(self):
        """Helper function to update the _optimized_pipeline field."""
        # Store the pipeline with the highest internal testing score
        if self._pareto_front:
            self._optimized_pipeline_score = -float('inf')
            for pipeline, pipeline_scores in zip(self._pareto_front.items, reversed(self._pareto_front.keys)):
                if pipeline_scores.wvalues[1] > self._optimized_pipeline_score:
                    self._optimized_pipeline = pipeline
                    self._optimized_pipeline_score = pipeline_scores.wvalues[1]

            if not self._optimized_pipeline:
                raise RuntimeError('There was an error in the TPOT optimization '
                                   'process. This could be because the data was '
                                   'not formatted properly, or because data for '
                                   'a regression problem was provided to the '
                                   'TPOTClassifier object. Please make sure you '
                                   'passed the data to TPOT correctly.')
            else:
                pareto_front_wvalues = [pipeline_scores.wvalues[1] for pipeline_scores in self._pareto_front.keys]
                if not self._last_optimized_pareto_front:
                    self._last_optimized_pareto_front = pareto_front_wvalues
                elif self._last_optimized_pareto_front == pareto_front_wvalues:
                    self._last_optimized_pareto_front_n_gens += 1
                else:
                    self._last_optimized_pareto_front = pareto_front_wvalues
                    self._last_optimized_pareto_front_n_gens = 0
        else:
            # If user passes CTRL+C in initial generation, self._pareto_front (halloffame) shoule be not updated yet.
            # need raise RuntimeError because no pipeline has been optimized
            raise RuntimeError('A pipeline has not yet been optimized. Please call fit() first.')