[Core] Raise when non-multi-instance DP clients target a DP rank (#19227)

Signed-off-by: Jon Swenson <jmswen@gmail.com>

tests/v1/engine/test_async_llm.py

@@ -250,3 +250,32 @@ async def test_customize_loggers(monkeypatch):
         assert len(engine.stat_loggers) == 1
         assert len(engine.stat_loggers[0]) == 1
         engine.stat_loggers[0][0].log.assert_called_once()
+
+
+@pytest.mark.asyncio(scope="module")
+async def test_dp_rank_argument(monkeypatch: pytest.MonkeyPatch):
+    with monkeypatch.context() as m, ExitStack() as after:
+        m.setenv("VLLM_USE_V1", "1")
+
+        engine = AsyncLLM.from_engine_args(TEXT_ENGINE_ARGS)
+        after.callback(engine.shutdown)
+
+        sampling_params = SamplingParams(max_tokens=100,
+                                         output_kind=RequestOutputKind.DELTA,
+                                         temperature=1.0,
+                                         seed=33)
+
+        # Test with valid DP rank.
+        async for _ in engine.generate(request_id="request-34",
+                                       prompt=TEXT_PROMPT,
+                                       sampling_params=sampling_params,
+                                       data_parallel_rank=0):
+            pass
+
+        # Test with out-of-range DP rank.
+        with pytest.raises(ValueError):
+            async for _ in engine.generate(request_id="request-35",
+                                           prompt=TEXT_PROMPT,
+                                           sampling_params=sampling_params,
+                                           data_parallel_rank=1):
+                pass