[Core][Distributed] add same-node detection (#5369)

tests/distributed/test_same_node.py

@@ -0,0 +1,11 @@
+import os
+
+import torch
+
+from vllm.distributed.parallel_state import is_in_the_same_node
+
+torch.distributed.init_process_group(backend="gloo")
+test_result = is_in_the_same_node(torch.distributed.group.WORLD)
+
+expected = os.environ.get("VLLM_TEST_SAME_HOST", "1") == "1"
+assert test_result == expected, f"Expected {expected}, got {test_result}"