[Bugfix][CPU] Fix thread num for shared memory communication (#33317)

Signed-off-by: jiang1.li <jiang1.li@intel.com>
Signed-off-by: Li, Jiang <bigpyj64@gmail.com>
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

vllm/distributed/device_communicators/cpu_communicator.py

@@ -205,10 +205,22 @@ class _CPUSHMDistributed:
         self.handle = self._init_cpu_shm()
 
     def _init_cpu_shm(self) -> int:
+        thread_num_tensor = torch.tensor(
+            [torch.get_num_threads()],
+            dtype=torch.int64,
+        )
+        torch.distributed.all_reduce(
+            thread_num_tensor,
+            op=torch.distributed.ReduceOp.MIN,
+            group=self.communicator.device_group,
+        )
+        thread_num = thread_num_tensor.item()
+
         handle = torch.ops._C.init_shm_manager(
             self.group_name,
             self.communicator.world_size,
             self.communicator.rank,
+            thread_num,
         )
         torch.distributed.barrier(self.communicator.device_group)
         torch.ops._C.join_shm_manager(