code: def barrier(use_mpi=False):
      def run_barrier(rank, use_mpi=False):
          dev = cuda.Device(rank)
          dev.use()
          comm = NCCLBackend(N_WORKERS, rank, use_mpi=use_mpi)
          comm.barrier()
          before = time.time()
          if rank == 0:
              time.sleep(2)
          comm.barrier()
          after = time.time()
          assert int(after - before) == 2
  
      if use_mpi:
          from mpi4py import MPI
  
          # This process was run with mpiexec
          run_barrier(MPI.COMM_WORLD.Get_rank(), True)
      else:
          _launch_workers(run_barrier)
  
imports: import sys
  import time
  import warnings
  import cupy
  from cupy import cuda
  from cupy.cuda import nccl
  from cupy import testing
  from cupyx.distributed import init_process_group
  from cupyx.distributed._nccl_comm import NCCLBackend
  from cupyx.distributed._store import ExceptionAwareProcess
  from cupyx.scipy import sparse
label_desc: Use image node_id 12 for calling a global function with example usage: barrier(use_mpi) without return types