code: def forward(self, x):
      return chainer.functions.sum(self.l(x))
  
file_path: chainer/tests/chainermn_tests/extensions_tests/test_observation_aggregator.py
imports: from __future__ import division
  import pytest
  import numpy
  import chainer
  import chainer.testing
  from chainer.training import extension
  from chainer.backend import cuda
  import chainermn
  from chainermn.testing import get_device
  from chainermn.extensions import ObservationAggregator
  import chainerx
label_desc: Use image node_id 2 for calling the DummyChain obj's underlying member method code with example usage: obj.forward(x) and returns: chainer