[Bugfix][LoRA] Fix the bug when runs Qwen3-Reranker-0.6B with LoRA. (#7156)

### What this PR does / why we need it?
Fix the error that reports while initializing qwen3-reranker-0.6b model
with `--enable-lora`.
And add a testcase to verify the fix.

- vLLM version: v0.17.0
- vLLM main:
4034c3d32e
---------
Signed-off-by: paulyu12 <507435917@qq.com>
Co-authored-by: Mengqing Cao <cmq0113@163.com>

tests/e2e/singlecard/pooling/template/qwen3_reranker.jinja

@@ -0,0 +1,11 @@
+<|im_start|>system
+Judge whether the Document meets the requirements based on the Query and the Instruct provided. Note that the answer can only be "yes" or "no".<|im_end|>
+<|im_start|>user
+<Instruct>: {{ messages | selectattr("role", "eq", "system") | map(attribute="content") | first | default("Given a web search query, retrieve relevant passages that answer the query") }}
+<Query>: {{ messages | selectattr("role", "eq", "query") | map(attribute="content") | first }}
+<Document>: {{ messages | selectattr("role", "eq", "document") | map(attribute="content") | first }}<|im_end|>
+<|im_start|>assistant
+<think>
+
+</think>
+

tests/e2e/singlecard/pooling/test_qwen3_reranker_lora.py

@@ -0,0 +1,73 @@
+from pathlib import Path
+
+from vllm import LLM
+
+model_name = "Qwen/Qwen3-Reranker-0.6B"
+
+
+def get_llm() -> LLM:
+    """
+    Initializes and returns the LLM model for Qwen3-Reranker.
+
+    Returns:
+        LLM: Configured vLLM instance for reranking tasks.
+
+    Note:
+        This function loads the ORIGINAL Qwen3-Reranker model with specific
+        overrides to make it compatible with vLLM's score API.
+    """
+    return LLM(
+        # Specify the original model from HuggingFace
+        model=model_name,
+        # Use pooling runner for score task
+        runner="pooling",
+        # HuggingFace model configuration overrides required for compatibility
+        hf_overrides={
+            # Manually route to sequence classification architecture
+            # This tells vLLM to use Qwen3ForSequenceClassification instead of
+            # the default Qwen3ForCausalLM
+            "architectures": ["Qwen3ForSequenceClassification"],
+            # Specify which token logits to extract from the language model head
+            # The original reranker uses "no" and "yes" token logits for scoring
+            "classifier_from_token": ["no", "yes"],
+            # Enable special handling for original Qwen3-Reranker models
+            # This flag triggers conversion logic that transforms the two token
+            # vectors into a single classification vector
+            "is_original_qwen3_reranker": True,
+        },
+        enable_lora=True,
+    )
+
+
+def test_reranker_models_lora():
+    # Load the Jinja template for formatting query-document pairs
+    # The template ensures proper formatting for the reranker model
+    template_home = Path(__file__).parent / "template"
+    template_path = "qwen3_reranker.jinja"
+    chat_template = (template_home / template_path).read_text()
+
+    # Sample queries for testing the reranker
+    queries = [
+        "What is the capital of China?",
+        "Explain gravity",
+    ]
+
+    # Corresponding documents to be scored against each query
+    documents = [
+        "The capital of China is Beijing.",
+        "Gravity is a force that attracts two bodies towards each other. It gives weight to physical objects and is responsible for the movement of planets around the sun.",
+    ]
+
+    # Initialize the LLM model with the original Qwen3-Reranker configuration
+    llm = get_llm()
+
+    # Compute relevance scores for each query-document pair
+    # The score() method returns a relevance score for each pair
+    # Higher scores indicate better relevance
+    outputs = llm.score(queries, documents, chat_template=chat_template)
+
+    # Extract and print the relevance scores from the outputs
+    # Each output contains a score representing query-document relevance
+    print("-" * 30)
+    print("Relevance scores:", [output.outputs.score for output in outputs])
+    print("-" * 30)