code: def get_group(self):
          
          if self.group is None:
              self.group = self.get_field('group')
              if self.group is not None:
                  # group data from LightGBM is boundaries data, need to convert to group size
                  self.group = np.diff(self.group)
          return self.group
docs: Get the group of the Dataset.
  
          Returns
          -------
          group : numpy array or None
              Group size of each group.