docs: def _calibrate(self, data, labels):
          """\
          Calibrate the network using `self._bmus`.
          """
          # network calibration
          classifier = defaultdict(Counter)
          for (i,j), label in zip(self._bmus, labels):
              classifier[i,j][label] += 1
          self.classifier = {}
          for ij, cnt in classifier.items():
              maxi = max(cnt.items(), key=itemgetter(1))
              nb = sum(cnt.values())
              self.classifier[ij] = maxi[0], maxi[1] / nb