code: def set_group(self, group):
          
          _check_call(_LIB.XGDMatrixSetGroup(self.handle,
                                             c_array(ctypes.c_uint, group),
                                             c_bst_ulong(len(group))))
docs: Set group size of DMatrix (used for ranking).
  
          Parameters
          ----------
          group : array like
              Group size of each group