code: def set_aggregation_method(self, aggregation_method: str):
      assert aggregation_method in AGGREGATION_METHODS
      self.aggregation_method = aggregation_method
  
imports: import copy
  import logging
  from itertools import product
  from typing import List, Optional, Iterator
  import numpy
  from gluonts.core.component import validated
  from gluonts.dataset.field_names import FieldName
  from gluonts.dataset.common import Dataset
  from gluonts.model.predictor import Predictor
  from gluonts.model.forecast import Forecast, SampleForecast
  from pts.model import PyTorchEstimator
  from pts import Trainer
  from .n_beats_estimator import NBEATSEstimator
  from .n_beats_network import VALID_LOSS_FUNCTIONS
label_desc: Use image node_id 2 for calling the NBEATSEnsemblePredictor obj's underlying member method code with example usage: obj.set_aggregation_method(aggregation_method) without return types