[Bugfix] Fix torch.compile() error when using MultiprocessingGPUExecutor

[zifeitong]

torch.compile() by default will use a process pool for async compiling which doesn't work with MultiprocessingGPUExecutor. Models uses torch.compile() like commandr therefore won't work:

[rank0]: Traceback (most recent call last):
[rank0]:   File "/var/data/home/vllm-upstream/benchmarks/benchmark_latency.py", line 226, in <module>
[rank0]:     main(args)
[rank0]:   File "/var/data/home/vllm-upstream/benchmarks/benchmark_latency.py", line 22, in main
[rank0]:     llm = LLM(model=args.model,
[rank0]:           ^^^^^^^^^^^^^^^^^^^^^
[rank0]:   File "/var/data/home/vllm-upstream/vllm/entrypoints/llm.py", line 143, in __init__
[rank0]:     self.llm_engine = LLMEngine.from_engine_args(
[rank0]:                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^
[rank0]:   File "/var/data/home/vllm-upstream/vllm/engine/llm_engine.py", line 359, in from_engine_args
[rank0]:     engine = cls(
[rank0]:              ^^^^
[rank0]:   File "/var/data/home/vllm-upstream/vllm/engine/llm_engine.py", line 222, in __init__
[rank0]:     self.model_executor = executor_class(
[rank0]:                           ^^^^^^^^^^^^^^^
[rank0]:   File "/var/data/home/vllm-upstream/vllm/executor/distributed_gpu_executor.py", line 25, in __init__
[rank0]:     super().__init__(*args, **kwargs)
[rank0]:   File "/var/data/home/vllm-upstream/vllm/executor/executor_base.py", line 41, in __init__
[rank0]:     self._init_executor()
[rank0]:   File "/var/data/home/vllm-upstream/vllm/executor/multiproc_gpu_executor.py", line 66, in _init_executor
[rank0]:     self._run_workers("load_model",
[rank0]:   File "/var/data/home/vllm-upstream/vllm/executor/multiproc_gpu_executor.py", line 123, in _run_workers
[rank0]:     ] + [output.get() for output in worker_outputs]
[rank0]:         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[rank0]:   File "/var/data/home/vllm-upstream/vllm/executor/multiproc_gpu_executor.py", line 123, in <listcomp>
[rank0]:     ] + [output.get() for output in worker_outputs]
[rank0]:          ^^^^^^^^^^^^
[rank0]:   File "/var/data/home/vllm-upstream/vllm/executor/multiproc_worker_utils.py", line 58, in get
[rank0]:     raise self.result.exception
[rank0]: AssertionError: daemonic processes are not allowed to have children
ERROR 06-03 20:34:21 multiproc_worker_utils.py:119] Worker VllmWorkerProcess pid 114600 died, exit code: -15

Setting TORCHINDUCTOR_COMPILE_THREADS to 1 can disable the async compiling process pool.

[youkaichao]

LGTM, thanks!