code: def setup_gpu(self, use_chx=False):
      self.comm = chainermn.create_communicator("flat")
      self.target = DynamicExampleModel()
      self.device = chainermn.testing.get_device(
          self.comm.intra_rank, use_chx
      )
      chainer.cuda.get_device_from_id(self.comm.intra_rank).use()
      self.target.to_device(self.device)
  
      self.target.a.W.data[:] = self.comm.rank
      self.target.b.W.data[:] = self.comm.rank + 1
      self.target.a.W.grad[:] = 0
      self.target.b.W.grad[:] = 0
      self.actual_optimizer = chainer.GradientMethod()
      self.actual_optimizer.create_update_rule = mock.MagicMock
  
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 2 for calling the TestMultiNodeOptimizerWithDynamicModel obj's underlying member method code with example usage: obj.setup_gpu(use_chx) without return types