[Frontend] API support for beam search (#9087)

Co-authored-by: youkaichao <youkaichao@126.com>

tests/entrypoints/openai/test_completion.py

@@ -495,25 +495,30 @@ async def test_batch_completions(client: openai.AsyncOpenAI, model_name: str):
         assert len(batch.choices) == 2
         assert batch.choices[0].text == batch.choices[1].text
 
-        # test n = 2
-        batch = await client.completions.create(
-            model=model_name,
-            prompt=prompts,
-            n=2,
-            max_tokens=5,
-            temperature=0.0,
-            extra_body=dict(
-                # NOTE: this has to be true for n > 1 in vLLM, but not necessary
-                # for official client.
-                use_beam_search=True),
-        )
-        assert len(batch.choices) == 4
-        assert batch.choices[0].text != batch.choices[
-            1].text, "beam search should be different"
-        assert batch.choices[0].text == batch.choices[
-            2].text, "two copies of the same prompt should be the same"
-        assert batch.choices[1].text == batch.choices[
-            3].text, "two copies of the same prompt should be the same"
+        try:
+            # test n = 2
+            batch = await client.completions.create(
+                model=model_name,
+                prompt=prompts,
+                n=2,
+                max_tokens=5,
+                temperature=0.0,
+                extra_body=dict(
+                    # NOTE: this has to be true for n > 1 in vLLM, but
+                    # not necessary for official client.
+                    use_beam_search=True),
+            )
+            assert len(batch.choices) == 4
+            assert batch.choices[0].text != batch.choices[
+                1].text, "beam search should be different"
+            assert batch.choices[0].text == batch.choices[
+                2].text, "two copies of the same prompt should be the same"
+            assert batch.choices[1].text == batch.choices[
+                3].text, "two copies of the same prompt should be the same"
+        except BadRequestError as e:
+            # the only allowed exception is when beam search is not supported
+            # in the default mqllmengine
+            assert "--disable-frontend-multiprocessing" in str(e)
 
         # test streaming
         batch = await client.completions.create(