code: def test_update_with_gpu(self, use_chx):
      self.setup_gpu(use_chx)
      self.optimizer = chainermn.create_multi_node_optimizer(
          self.actual_optimizer, self.comm
      )
      opt = self.optimizer.setup(self.target)
      assert opt is self.optimizer
      self.optimizer.update()
      assert self.actual_optimizer.t == 0
  
      self.optimizer.target.a.W.grad[:] = self.comm.rank
      self.optimizer.target.b.W.grad[:] = self.comm.rank + 1
      self.optimizer.target.c.W.grad[:] = self.comm.rank + 2
  
      self.optimizer.update()
      assert self.actual_optimizer.t == 1
      self.optimizer.target.a.W.update_rule.update.assert_called_once_with(
          self.optimizer.target.a.W
      )
      self.optimizer.target.b.W.update_rule.update.assert_called_once_with(
          self.optimizer.target.b.W
      )
      self.optimizer.target.c.W.update_rule.update.assert_called_once_with(
          self.optimizer.target.c.W
      )
  
      base = (self.comm.size - 1.0) / 2
      chainer.testing.assert_allclose(
          self.optimizer.target.a.W.grad, (base + 0) * np.ones((3, 2))
      )
      chainer.testing.assert_allclose(
          self.optimizer.target.b.W.grad, (base + 1) * np.ones((4, 3))
      )
      chainer.testing.assert_allclose(
          self.optimizer.target.c.W.grad, (base + 2) * np.ones((5, 4))
      )
  
imports: import chainer
  import chainer.testing
  import chainer.testing.attr
  import chainermn
  import chainermn.testing
  import mock
  import numpy
  import pytest
label_desc: Use image node_id 4 for calling the TestMultiNodeOptimizer obj's underlying member method code with example usage: obj.test_update_with_gpu(use_chx) without return types