[Core][Bugfix] Fix Online MM Beam Search (vllm-project#19688)

Signed-off-by: Alex-Brooks <[email protected]>

Author: alex-jw-brooks

tests/entrypoints/openai/test_vision.py

@@ -25,6 +25,25 @@
     "https://upload.wikimedia.org/wikipedia/commons/0/0b/RGBA_comp.png",
 ]
 
+EXPECTED_MM_BEAM_SEARCH_RES = [
+    [
+        "The image shows a wooden boardwalk leading through a",
+        "The image shows a wooden boardwalk extending into a",
+    ],
+    [
+        "The image shows two parrots perched on",
+        "The image shows two birds perched on a cur",
+    ],
+    [
+        "The image shows a Venn diagram with three over",
+        "This image shows a Venn diagram with three over",
+    ],
+    [
+        "This image displays a gradient of colors ranging from",
+        "This image displays a gradient of colors transitioning from",
+    ],
+]
+
 
 @pytest.fixture(scope="module")
 def server():
@@ -270,10 +289,13 @@ async def test_single_chat_session_image_base64encoded(
 
 @pytest.mark.asyncio
 @pytest.mark.parametrize("model_name", [MODEL_NAME])
-@pytest.mark.parametrize("image_url", TEST_IMAGE_URLS)
+@pytest.mark.parametrize("image_idx", list(range(len(TEST_IMAGE_URLS))))
 async def test_single_chat_session_image_base64encoded_beamsearch(
-        client: openai.AsyncOpenAI, model_name: str, image_url: str,
+        client: openai.AsyncOpenAI, model_name: str, image_idx: int,
         base64_encoded_image: dict[str, str]):
+    # NOTE: This test also validates that we pass MM data through beam search
+    image_url = TEST_IMAGE_URLS[image_idx]
+    expected_res = EXPECTED_MM_BEAM_SEARCH_RES[image_idx]
 
     messages = [{
         "role":
@@ -297,10 +319,11 @@ async def test_single_chat_session_image_base64encoded_beamsearch(
         messages=messages,
         n=2,
         max_completion_tokens=10,
+        temperature=0.0,
         extra_body=dict(use_beam_search=True))
     assert len(chat_completion.choices) == 2
-    assert chat_completion.choices[
-        0].message.content != chat_completion.choices[1].message.content
+    for actual, expected_str in zip(chat_completion.choices, expected_res):
+        assert actual.message.content == expected_str
 
 
 @pytest.mark.asyncio

vllm/engine/protocol.py

@@ -88,9 +88,18 @@ async def beam_search(
         if processed_inputs["type"] == "embeds":
             raise NotImplementedError
 
-        prompt_token_ids = processed_inputs["prompt_token_ids"]
+        # This is a workaround to fix multimodal beam search; this is a
+        # bandaid fix for 2 small problems:
+        # 1. Multi_modal_data on the processed_inputs currently resolves to
+        #    `None`.
+        # 2. preprocessing above expands the multimodal placeholders. However,
+        #    this happens again in generation, so the double expansion causes
+        #    a mismatch.
+        # TODO - would be ideal to handle this more gracefully.
+        prompt_token_ids = prompt.get("prompt_token_ids")
+        multi_modal_data = prompt.get("multi_modal_data")
+
         prompt_text = processed_inputs.get("prompt")
-        multi_modal_data = processed_inputs.get("multi_modal_data")
         mm_processor_kwargs = processed_inputs.get("mm_processor_kwargs")
 
         tokenized_length = len(prompt_token_ids)

vllm/entrypoints/llm.py

@@ -15,7 +15,8 @@
 from typing_extensions import TypeVar, deprecated
 
 from vllm.beam_search import (BeamSearchInstance, BeamSearchOutput,
-                              BeamSearchSequence, get_beam_search_score)
+                              BeamSearchSequence,
+                              create_sort_beams_key_function)
 from vllm.config import (CompilationConfig, ModelDType, TokenizerMode,
                          is_init_field)
 from vllm.engine.arg_utils import (EngineArgs, HfOverrides, PoolerConfig,
@@ -575,10 +576,11 @@ def beam_search(
         lora_requests = self._get_beam_search_lora_requests(
             lora_request, prompts)
 
-        def sort_beams_key(x: BeamSearchSequence) -> float:
-            return get_beam_search_score(x.tokens, x.cum_logprob,
-                                         tokenizer.eos_token_id,
-                                         length_penalty)
+        tokenizer = self.get_tokenizer()
+        sort_beams_key = create_sort_beams_key_function(
+            tokenizer.eos_token_id,
+            length_penalty,
+        )
 
         def create_tokens_prompt_from_beam(
                 beam: BeamSearchSequence) -> TokensPrompt:
@@ -593,7 +595,6 @@ def create_tokens_prompt_from_beam(
                     "mm_processor_kwargs"] = beam.mm_processor_kwargs
             return TokensPrompt(**token_prompt_kwargs)
 
-        tokenizer = self.get_tokenizer()
         # generate 2 * beam_width candidates at each step
         # following the huggingface transformers implementation
         # at https://github.com/huggingface/transformers/blob/e15687fffe5c9d20598a19aeab721ae0a7580f8a/src/transformers/generation/beam_search.py#L534 # noqa