[bugfix] respect distributed_executor_backend in world_size=1 (#12934)

Signed-off-by: youkaichao <youkaichao@gmail.com>

vllm/config.py

@@ -1401,6 +1401,9 @@ class ParallelConfig:
             logger.info("Defaulting to use %s for distributed inference",
                         backend)
 
+        if self.distributed_executor_backend is None and self.world_size == 1:
+            self.distributed_executor_backend = "uni"
+
         self._verify_args()
 
     @property