code: def set_loss_fn(self, loss_fn):
      self.loss_fn = loss_fn
  
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 3 for calling the Aggregator obj's underlying member method code with example usage: obj.set_loss_fn(loss_fn) without return types