code: def test_is_last_hook(self):
      process_group = self._get_process_group_nccl()
  
      def hook(flags, bucket):
          flags.append(bucket.is_last())
          fut = torch.futures.Future()
          fut.set_result(bucket.buffer())
          return fut
  
      flags = []
      device_id = gpus_for_rank(self.world_size)[self.rank][0]
      model = nn.Sequential(
          nn.Linear(2, 4000, bias=False),
          *[nn.Linear(4000, 4000, bias=False) for _ in range(10)],
      )
      gpu_model = DistributedDataParallel(
          model.to(device_id),
          device_ids=[device_id],
          process_group=process_group,
      )
      gpu_model.register_comm_hook(state=flags, hook=hook)
      input = torch.randn(10, 2)
      gpu_model(input).sum().backward()
      self.assertTrue(flags[-1])
      self.assertFalse(any(flags[:-1]))
  
imports: import os
  import sys
  import torch
  from torch import nn
  import torch.distributed
  from torch.distributed.algorithms.ddp_comm_hooks import DDPCommHookType, register_ddp_comm_hook
  from torch.nn.parallel import DistributedDataParallel
  from torch.testing._internal.common_distributed import MultiProcessTestCase, requires_nccl, skip_if_lt_x_gpu
  from torch.testing._internal.common_utils import run_tests, TEST_WITH_DEV_DBG_ASAN
label_desc: Use image node_id 13 for calling the DistributedDataParallelCommHookTest obj's underlying member method code with example usage: obj.test_is_last_hook() and returns: fut