[Bugfix][CPU] Skip set_num_threads after thread binding (#38535)

Signed-off-by: jiang1.li <jiang1.li@intel.com>

.buildkite/scripts/hardware_ci/run-cpu-distributed-smoke-test.sh

 #!/bin/bash
 set -euox pipefail
 export VLLM_CPU_CI_ENV=0
+export VLLM_CPU_KVCACHE_SPACE=1 # avoid OOM
 
 echo "--- PP+TP"
-vllm serve meta-llama/Llama-3.2-3B-Instruct -tp=2 -pp=2 &
+vllm serve meta-llama/Llama-3.2-3B-Instruct -tp=2 -pp=2 --max-model-len=4096 &
 server_pid=$!
 timeout 600 bash -c "until curl localhost:8000/v1/models > /dev/null 2>&1; do sleep 1; done" || exit 1
 vllm bench serve \
@@ -23,7 +24,7 @@ if [ "$failed_req" -ne 0 ]; then
 fi
 
 echo "--- DP+TP"
-vllm serve meta-llama/Llama-3.2-3B-Instruct -tp=2 -dp=2 &
+vllm serve meta-llama/Llama-3.2-3B-Instruct -tp=2 -dp=2 --max-model-len=4096 &
 server_pid=$!
 timeout 600 bash -c "until curl localhost:8000/v1/models > /dev/null 2>&1; do sleep 1; done" || exit 1
 vllm bench serve \

vllm/v1/worker/cpu_worker.py

@@ -108,6 +108,15 @@ class CPUWorker(Worker):
             if ret:
                 logger.info(ret)
 
+        # After the thread binding, changing thread num is not allowed
+        def skip_set_num_threads(x: int):
+            logger.warning(
+                "CPU backend doesn't allow to use "
+                "`torch.set_num_threads` after the thread binding, skip it."
+            )
+
+        torch.set_num_threads = skip_set_num_threads
+
         # Note: unique identifier for creating allreduce shared memory
         os.environ["VLLM_DIST_IDENT"] = self.distributed_init_method.split(":")[-1]
         # Initialize the distributed environment.