[VLM] Generalized prompt updates for multi-modal processor (vllm-project#13964)

Signed-off-by: DarkLight1337 <[email protected]>
Signed-off-by: Louis Ulmer <[email protected]>

Author: DarkLight1337

docs/source/contributing/model/multimodal.md

@@ -720,13 +720,13 @@ def _get_mm_fields_config(
 
 :::::
 
-### Prompt replacements
+### Prompt updates
 
-Override {meth}`~vllm.multimodal.processing.BaseMultiModalProcessor._get_prompt_replacements` to
-return a list of {class}`~vllm.multimodal.processing.PromptReplacement` instances.
+Override {meth}`~vllm.multimodal.processing.BaseMultiModalProcessor._get_prompt_updates` to
+return a list of {class}`~vllm.multimodal.processing.PromptUpdate` instances.
 
-Each {class}`~vllm.multimodal.processing.PromptReplacement` instance specifies a find-and-replace
-operation performed by the HF processor.
+Each {class}`~vllm.multimodal.processing.PromptUpdate` instance specifies an update operation
+(e.g.: insertion, replacement) performed by the HF processor.
 
 ::::{tab-set}
 :::{tab-item} Basic example: LLaVA
@@ -743,15 +743,15 @@ for sample in text:
 ```
 
 It simply repeats each input `image_token` a number of times equal to the number of placeholder feature tokens (`num_image_tokens`).
-Based on this, we override {meth}`~vllm.multimodal.processing.BaseMultiModalProcessor._get_prompt_replacements` as follows:
+Based on this, we override {meth}`~vllm.multimodal.processing.BaseMultiModalProcessor._get_prompt_updates` as follows:
 
 ```python
-def _get_prompt_replacements(
+def _get_prompt_updates(
     self,
     mm_items: MultiModalDataItems,
     hf_processor_mm_kwargs: Mapping[str, object],
     out_mm_kwargs: MultiModalKwargs,
-) -> list[PromptReplacement]:
+) -> Sequence[PromptUpdate]:
     hf_config = self.info.get_hf_config()
     image_token_id = hf_config.image_token_index
 
@@ -859,7 +859,7 @@ prompt_tokens, prompts_length = _tokenize_prompts_with_image_and_batch(
 )
 ```
 
-To accommodate this, instead of a string you can return an instance of `PromptReplacementDetails`
+To accommodate this, instead of a string you can return an instance of `PromptUpdateDetails`
 with different `full` and `feature` attributes:
 
 ```python
@@ -878,7 +878,7 @@ def get_replacement_fuyu(item_idx: int):
     image_tokens = ([_IMAGE_TOKEN_ID] * ncols +
                     [_NEWLINE_TOKEN_ID]) * nrows
 
-    return PromptReplacementDetails(
+    return PromptUpdateDetails(
         full=image_tokens + [bos_token_id],
         features=image_tokens,
     )
@@ -888,12 +888,12 @@ Finally, noticing that the HF processor removes the `|ENDOFTEXT|` token from the
 we can search for it to conduct the replacement at the start of the string:
 
 ```python
-def _get_prompt_replacements(
+def _get_prompt_updates(
     self,
     mm_items: MultiModalDataItems,
     hf_processor_mm_kwargs: Mapping[str, object],
     out_mm_kwargs: MultiModalKwargs,
-) -> list[PromptReplacement]:
+) -> Sequence[PromptUpdate]:
     hf_config = self.info.get_hf_config()
     bos_token_id = hf_config.bos_token_id
     assert isinstance(bos_token_id, int)
@@ -913,7 +913,7 @@ def _get_prompt_replacements(
         image_tokens = ([_IMAGE_TOKEN_ID] * ncols +
                         [_NEWLINE_TOKEN_ID]) * nrows
 
-        return PromptReplacementDetails(
+        return PromptUpdateDetails(
             full=image_tokens + [bos_token_id],
             features=image_tokens,
         )

docs/source/design/mm_processing.md

@@ -6,11 +6,16 @@ To enable various optimizations in vLLM such as [chunked prefill](#chunked-prefi
 
 Here are the main features of {class}`~vllm.multimodal.processing.BaseMultiModalProcessor`:
 
-## Prompt Replacement Detection
+## Prompt Update Detection
 
-One of the main responsibilies of HF processor is to replace input placeholder tokens (e.g. `<image>` for a single image) with feature placeholder tokens (e.g. `<image><image>...<image>`, the number of which equals to the feature size). The information about which tokens have been replaced is key to finding the correspondence between placeholder feature tokens and multi-modal inputs.
+One of the main responsibilies of HF processor is to update the prompt with placeholder tokens. For example:
 
-In vLLM, this information is specified using {class}`~vllm.multimodal.processing.PromptReplacement` in {meth}`~vllm.multimodal.processing.BaseMultiModalProcessor._get_prompt_replacements`. Given this specification, we can automatically detect whether HF has replaced the input placeholder tokens by checking whether the feature placeholder tokens exist in the prompt.
+- Insert feature placeholder tokens (e.g. `<image><image>...<image>`, the number of which equals to the feature size) at the start of the string.
+- Replace existing input placeholder tokens (e.g. `<image>` for a single image) with feature placeholder tokens (e.g. `<image><image>...<image>`, the number of which equals to the feature size).
+
+The information about which tokens have been updated is key to finding the correspondence between placeholder feature tokens and multi-modal inputs.
+
+In vLLM, this information is specified using {class}`~vllm.multimodal.processing.PromptUpdate` in {meth}`~vllm.multimodal.processing.BaseMultiModalProcessor._get_prompt_updates`. We can automatically detect whether HF has updated the prompt by checking the existence of the updated tokens.
 
 ## Tokenized Prompt Inputs
 
@@ -22,7 +27,7 @@ Consider that HF processors follow these main steps:
 
 1. Tokenize the text
 2. Process multi-modal inputs
-3. Perform prompt replacement
+3. Perform prompt updates
 
 And we require that:
 
@@ -44,21 +49,21 @@ Moreover, since the tokenized text has not passed through the HF processor, we h
 
 We work around the first issue by requiring each model to define how to generate dummy text based on the number of multi-modal inputs, via {meth}`~vllm.multimodal.profiling.BaseDummyInputsBuilder.get_dummy_processor_inputs`. This lets us generate dummy text corresponding to the multi-modal inputs and input them together to obtain the processed multi-modal data.
 
-(mm-automatic-prompt-replacement)=
+(mm-automatic-prompt-updating)=
 
-### Automatic prompt replacement
+### Automatic prompt updating
 
 We address the second issue by implementing model-agnostic code in
-{meth}`~vllm.multimodal.processing.BaseMultiModalProcessor._apply_prompt_replacements` to automatically replace input placeholder tokens with feature placeholder tokens based on the specification outputted by {meth}`~vllm.multimodal.processing.BaseMultiModalProcessor._get_prompt_replacements`.
+{meth}`~vllm.multimodal.processing.BaseMultiModalProcessor._apply_prompt_updates` to automatically update the prompt with feature placeholder tokens based on the specification outputted by {meth}`~vllm.multimodal.processing.BaseMultiModalProcessor._get_prompt_updates`.
 
 ### Summary
 
-With the help of dummy text and automatic prompt replacement, our multi-modal processor can finally accept both text and token prompts with multi-modal data. The detailed logic is shown in {meth}`~vllm.multimodal.processing.BaseMultiModalProcessor._apply_hf_processor_main`.
+With the help of dummy text and automatic prompt updating, our multi-modal processor can finally accept both text and token prompts with multi-modal data. The detailed logic is shown in {meth}`~vllm.multimodal.processing.BaseMultiModalProcessor._apply_hf_processor_main`.
 
 ## Processor Output Caching
 
 Some HF processors, such as the one for Qwen2-VL, are [very slow](gh-issue:9238). To alleviate this problem, we cache the multi-modal outputs of HF processor to avoid processing the same multi-modal input (e.g. image) again.
 
 When new data is passed in, we first check which items are in the cache, and which ones are missing. The missing items are passed into the HF processor in a single batch and cached, before being merged with the existing items in the cache.
 
-Since we only process the missing multi-modal data items, the number of input placeholder tokens no longer corresponds to the number of the multi-modal inputs, so they can't be passed alongside the text prompt to HF processor. Therefore, we process the text and multi-modal inputs separately, using [dummy text](#mm-dummy-text) to avoid HF errors. Since this skips HF's prompt replacement code, we apply [automatic prompt replacement](#mm-automatic-prompt-replacement) afterwards to keep the output tokens and multi-modal data consistent with each other.
+Since we only process the missing multi-modal data items, the number of input placeholder tokens no longer corresponds to the number of the multi-modal inputs, so they can't be passed alongside the text prompt to HF processor. Therefore, we process the text and multi-modal inputs separately, using [dummy text](#mm-dummy-text) to avoid HF errors. Since this skips HF's prompt updating code, we apply [automatic prompt updating](#mm-automatic-prompt-updating) afterwards to keep the output tokens and multi-modal data consistent with each other.