[Misc] Revive to use loopback address for driver IP (#7091)

Signed-off-by: Rui Qiao <ruisearch42@gmail.com>

vllm/executor/ray_gpu_executor.py

@@ -216,6 +216,16 @@ class RayGPUExecutor(DistributedGPUExecutor):
         self._run_workers("update_environment_variables",
                           all_args=all_args_to_update_environment_variables)
 
+        if len(node_gpus) == 1:
+            # in single node case, we don't need to get the IP address.
+            # the loopback address is sufficient
+            # NOTE: a node may have several IP addresses, one for each
+            # network interface. `get_ip()` might return any of them,
+            # while they might not work for communication inside the node
+            # if the network setup is complicated. Using the loopback address
+            # solves this issue, as it always works for communication inside
+            # the node.
+            driver_ip = "127.0.0.1"
         distributed_init_method = get_distributed_init_method(
             driver_ip, get_open_port())