[Frontend] Support using chat template as custom score template for reranking models (#30550)

Signed-off-by: Jakub Zakrzewski <jzakrzewski@nvidia.com>
Signed-off-by: wang.yuqi <yuqi.wang@daocloud.io>
Signed-off-by: wang.yuqi <noooop@126.com>
Co-authored-by: wang.yuqi <yuqi.wang@daocloud.io>

docs/serving/openai_compatible_server.md

@@ -669,6 +669,21 @@ You can find the documentation for cross encoder models at [sbert.net](https://w
 
 Code example: [examples/pooling/score/openai_cross_encoder_score.py](../../examples/pooling/score/openai_cross_encoder_score.py)
 
+#### Score Template
+
+Some scoring models require a specific prompt format to work correctly. You can specify a custom score template using the `--chat-template` parameter (see [Chat Template](#chat-template)).
+
+Score templates are supported for **cross-encoder** models only. If you are using an **embedding** model for scoring, vLLM does not apply a score template.
+
+Like chat templates, the score template receives a `messages` list. For scoring, each message has a `role` attribute—either `"query"` or `"document"`. For the usual kind of point-wise cross-encoder, you can expect exactly two messages: one query and one document. To access the query and document content, use Jinja's `selectattr` filter:
+
+- **Query**: `{{ (messages | selectattr("role", "eq", "query") | first).content }}`
+- **Document**: `{{ (messages | selectattr("role", "eq", "document") | first).content }}`
+
+This approach is more robust than index-based access (`messages[0]`, `messages[1]`) because it selects messages by their semantic role. It also avoids assumptions about message ordering if additional message types are added to `messages` in the future.
+
+Example template file: [examples/pooling/score/template/nemotron-rerank.jinja](../../examples/pooling/score/template/nemotron-rerank.jinja)
+
 #### Single inference
 
 You can pass a string to both `text_1` and `text_2`, forming a single sentence pair.