update num_inst and sum_metric

def _update(self):
        """ update num_inst and sum_metric """
        aps = []
        for k, v in self.records.items():
            recall, prec = self._recall_prec(v, self.counts[k])
            ap = self._average_precision(recall, prec)
            aps.append(ap)
            if self.num is not None and k < (self.num - 1):
                self.sum_metric[k] = ap
                self.num_inst[k] = 1
        if self.num is None:
            self.num_inst = 1
            self.sum_metric = np.mean(aps)
        else:
            self.num_inst[-1] = 1
            self.sum_metric[-1] = np.mean(aps)