code: def out_of_bag_mae(self):
          
          if not self._out_of_bag_mae_clean:
              try:
                  self._out_of_bag_mae = self.test(self.out_of_bag_samples)
                  self._out_of_bag_mae_clean = True
              except NodeNotReadyToPredict:
                  return
          return self._out_of_bag_mae.copy()
docs: Returns the mean absolute error for predictions on the out-of-bag
          samples.