[Core][Dsitributed] fix del exception

[youkaichao]

Exception warnings found in main branch:

https://buildkite.com/vllm/ci/builds/6086#018f2c52-88b7-4d39-a948-1529eca365e8

tests/distributed/test_chunked_prefill_distributed.py::test_models[16-5-half-meta-llama/Llama-2-7b-hf]
/usr/local/lib/python3.10/dist-packages/_pytest/unraisableexception.py:80: PytestUnraisableExceptionWarning: Exception ignored in: <function NCCLCommunicator.del at 0x7f6ce2d739a0>
Traceback (most recent call last):
File "/usr/local/lib/python3.10/dist-packages/vllm/distributed/device_communicators/pynccl.py", line 285, in del
dist.destroy_process_group()
File "/usr/local/lib/python3.10/dist-packages/torch/distributed/distributed_c10d.py", line 1656, in destroy_process_group
assert pg is not None
AssertionError

Try to see if this fixes the exception.

[youkaichao]

@simon-mo @zhuohan123 this is even trickier than I thought. After I remove the code that raised the exception, the test hangs for half an hour: https://buildkite.com/vllm/ci/builds/6120#018f2d8c-dc83-4595-bd26-68eb581d89fa

It is quite a coincidence that the current ci works, because the exception happens before the deadlock :(

I will try to figure out what happened ...