code: def check_bcast(x, communicator):
      root = 0
      if communicator.rank == root:
          y = chainermn.functions.bcast(communicator, x, root)
      else:
          y = chainermn.functions.bcast(communicator, None, root)
      e = chainer.functions.mean_squared_error(y, x)
      e.backward()
  
      # Check backward does not fall in deadlock, and error = 0 in root.
      if communicator.rank == root:
          assert 0 == e.data
          assert None is not x.grad
  
imports: import chainer
  import chainer.testing
  import chainer.testing.attr
  import numpy
  import pytest
  import chainermn
  import chainermn.functions
label_desc: Use image node_id 8 for calling a global function with example usage: check_bcast(x, communicator) without return types