Use an env var SGLANG_SET_CPU_AFFINITY to set cpu affinity; turn it off by default

[merrymercy]

It will conflict with some other libraries (e.g. Ray). We see an error message like below, so we want to turn it off by default.

Traceback (most recent call last):
  File "/usr/lib/python3.10/multiprocessing/process.py", line 314, in _bootstrap
    self.run()
  File "/usr/lib/python3.10/multiprocessing/process.py", line 108, in run
    self._target(*self._args, **self._kwargs)
  File "/data/dhou/runner/actions-runner/_work/sglang-private/sglang-private/python/sglang/srt/managers/scheduler.py", line 1526, in run_scheduler_process
    gpu_proc_affinity(server_args.tp_size, server_args.nnodes, gpu_id)
  File "/data/dhou/runner/actions-runner/_work/sglang-private/sglang-private/python/sglang/srt/utils.py", line 1053, in gpu_proc_affinity
    p.cpu_affinity(bind_cpu_ids)
  File "/usr/local/lib/python3.10/dist-packages/ray/thirdparty_files/psutil/__init__.py", line [89](https://github.com/xai-org/sglang-private/actions/runs/12041611115/job/33575335155#step:4:90)3, in cpu_affinity
    self._proc.cpu_affinity_set(list(set(cpus)))
  File "/usr/local/lib/python3.10/dist-packages/ray/thirdparty_files/psutil/_pslinux.py", line 1717, in wrapper
    return fun(self, *args, **kwargs)
  File "/usr/local/lib/python3.10/dist-packages/ray/thirdparty_files/psutil/_pslinux.py", line 2234, in cpu_affinity_set
    raise ValueError(
ValueError: CPU number 96 is not eligible; choose between [4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73]
Process Process-6:
Traceback (most recent call last):
  File "/usr/local/lib/python3.10/dist-packages/ray/thirdparty_files/psutil/_pslinux.py", line 2222, in cpu_affinity_set
    cext.proc_cpu_affinity_set(self.pid, cpus)
OSError: [Errno 22] Invalid argument

[HaiShaw]

@merrymercy Just curious - how to reproduce the error that you saw? will have a further look.