[Doc] Add docs for prompt replacement (#12318)

Signed-off-by: DarkLight1337 <tlleungac@connect.ust.hk>

vllm/model_executor/models/ultravox.py

@@ -218,7 +218,7 @@ class UltravoxMultiModalProcessor(
         return [
             PromptReplacement(
                 modality="audio",
-                target='<|audio|>',
+                target="<|audio|>",
                 replacement=get_replacement_ultravox,
             )
         ]

vllm/multimodal/processing.py

@@ -29,41 +29,101 @@ if TYPE_CHECKING:
 logger = init_logger(__name__)
 
 _S = TypeVar("_S", str, list[int])
-_PromptSeq = Union[str, list[int]]
+
+PromptSeq = Union[str, list[int]]
+"""A token sequence (list of token IDs) or text."""
 
 
 @dataclass
 class PromptReplacementDetails:
-    full: _PromptSeq
+    """Details about the replacement token sequence or text."""
+
+    full: PromptSeq
     """The full replacement."""
 
-    features: _PromptSeq
+    features: PromptSeq
     """
-    The part of the replacement that corresponds to placeholder feature tokens.
+    The part of the replacement that corresponds to feature placeholders;
+    this will be replaced by the output of the vision encoder during model
+    inference.
     """
 
     @staticmethod
-    def from_seq(seq: _PromptSeq) -> "PromptReplacementDetails":
+    def from_seq(seq: PromptSeq) -> "PromptReplacementDetails":
         return PromptReplacementDetails(full=seq, features=seq)
 
 
-_PromptRepl = Union[_PromptSeq, PromptReplacementDetails]
+PromptRepl = Union[PromptSeq, PromptReplacementDetails]
+"""
+The replacement token sequence or text.
+
+If only part of the replacement corresponds to feature placeholders, you can
+use :class:`PromptReplacementDetails` to specify which part.
+"""
 
 
 @dataclass
 class PromptReplacement:
     """
     Defines how to replace portions of an input prompt with placeholder tokens.
+
+    Example:
+
+        For each image, replace one ``<image>`` input placeholder in the prompt
+        with a number of ``<image>`` feature placeholders
+        equal to the feature size of the vision encoder:
+
+        .. code-block:: python
+
+            PromptReplacement(
+                modality="image",
+                target="<image>",
+                replacement="<image>" * image_feature_size,
+            )
+
+        As above, but further pad the feature placeholders with ``<image_bos>``
+        and `<image_eos>``, which are not supposed to be passed to the vision
+        encoder:
+
+        .. code-block:: python
+
+            PromptReplacement(
+                modality="image",
+                target="<image>",
+                replacement=PromptReplacementDetails(
+                    full="".join([
+                        "<image_bos>",
+                        "<image>" * image_feature_size,
+                        "<image_eos>",
+                    ]),
+                    features="<image>" * image_feature_size,
+                ),
+            )
+
+        To avoid unnecessary tokenization during prompt replacement,
+        we recommended passing token sequences instead of text:
+
+        .. code-block:: python
+
+            PromptReplacement(
+                modality="image",
+                target=[image_token_id],
+                replacement=PromptReplacementDetails(
+                    full=([image_bos_id] + [image_token_id] * image_feature_size
+                          + [image_eos_id]),
+                    features=[image_token_id] * image_feature_size,
+                ),
+            )
     """
 
     modality: str
     """The modality for which the replacement is made."""
 
-    target: _PromptSeq
+    target: PromptSeq
     """The token sequence (or text) to find and replace."""
 
-    replacement: Union[Callable[[int], _PromptRepl],
-                       _PromptRepl] = field(repr=False)
+    replacement: Union[Callable[[int], PromptRepl],
+                       PromptRepl] = field(repr=False)
     """
     Given the index of the processed item within :attr:`modality`,
     output the replacement token sequence (or text).
@@ -126,6 +186,10 @@ def full_groupby_modality(values: Iterable[_M]) -> ItemsView[str, list[_M]]:
 
 @dataclass
 class _BoundPromptSequence:
+    """
+    A :data:`_PromptSeq` bound to a tokenizer to automatically
+    convert between token sequence and text representations.
+    """
     tokenizer: AnyTokenizer = field(repr=False)
 
     _text: Optional[str]
@@ -134,7 +198,7 @@ class _BoundPromptSequence:
     @staticmethod
     def from_seq(
         tokenizer: AnyTokenizer,
-        seq: _PromptSeq,
+        seq: PromptSeq,
     ) -> "_BoundPromptSequence":
         return _BoundPromptSequence(
             tokenizer=tokenizer,
@@ -180,9 +244,9 @@ class BoundPromptReplacement:
     tokenizer: AnyTokenizer = field(repr=False)
     modality: str
 
-    _target: _PromptSeq
-    _replacement: Union[Callable[[int], _PromptRepl],
-                        _PromptRepl] = field(repr=False)
+    _target: PromptSeq
+    _replacement: Union[Callable[[int], PromptRepl],
+                        PromptRepl] = field(repr=False)
 
     def __post_init__(self) -> None:
         self._replacement_cache = dict[int, _BoundPromptReplacementGroup]()
@@ -350,7 +414,7 @@ def find_text_matches(
 
 
 def _resolve_matches(
-    prompt: _PromptSeq,
+    prompt: PromptSeq,
     mm_matches: Mapping[str, Sequence[_PromptReplacementMatch]],
 ) -> list[_PromptReplacementMatch]:
     """