code: def set_optimizer(self, optimizer_cls, optimizer_args):
      if len(list(self.model.trainable_variables)) == 0:
          self.optimizer = None
          return
      self.optimizer = optimizer_cls(**optimizer_args)
  
file_path: BigDL/python/ppml/src/bigdl/ppml/fl/nn/tensorflow/aggregator.py
imports: import os
  from bigdl.ppml.utils.safepickle import SafePickle
  import logging
  import threading
  from bigdl.dllib.utils.log4Error import invalidInputError, invalidOperationError
  from bigdl.ppml.fl.nn.utils import ndarray_map_to_tensor_map
  from threading import Condition
  import numpy
  import tensorflow
label_desc: Use image node_id 4 for calling the Aggregator obj's underlying member method code with example usage: obj.set_optimizer(optimizer_cls, optimizer_args) without return types