fix : MMLU eval 使用 chat template 格式匹配 SFT 训练数据

- 原 prompt 为纯文本格式，与 SFT chat template 不匹配导致模型输出随机
- 新增 apply_chat() 将 MMLU prompt 包装为 user/assistant 对话格式
- choice_text 改为单字母（去掉空格前缀）适配模板输出
- 5-shot 时 few-shot 示例作为独立 user/assistant 轮次插入

scripts/tools/evaluate_mmlu.py

@@ -157,10 +157,32 @@ def build_prompt(
     return prompt
 
 
+def apply_chat(
+    tokenizer, raw_prompt: str, n_shot: int, dev_data: list[dict] | None
+) -> str:
+    """Wrap raw MMLU prompt in the model's chat template format.
+
+    For few-shot, prepend example Q&A pairs as a second user/assistant exchange.
+    """
+    messages = []
+    if n_shot > 0 and dev_data:
+        for item in dev_data[:n_shot]:
+            q = f"Question: {item['question']}\n"
+            for k in ("A", "B", "C", "D"):
+                q += f"{k}. {item[k]}\n"
+            q += "Answer:"
+            messages.append({"role": "user", "content": q})
+            messages.append({"role": "assistant", "content": item["answer"]})
+    messages.append({"role": "user", "content": raw_prompt})
+    return tokenizer.apply_chat_template(
+        messages, tokenize=False, add_generation_prompt=True
+    )
+
+
 def choice_logprob(
     model, tokenizer, context_ids: list[int], choice_letter: str, device: str
 ) -> float:
-    choice_text = f" {choice_letter}"
+    choice_text = choice_letter
     choice_ids = tokenizer.encode(choice_text, add_special_tokens=False)
     input_ids = context_ids + choice_ids
     max_len = model.config.max_len
@@ -196,8 +218,11 @@ def evaluate_subject(
     correct = 0
     total = 0
     for item in tqdm.tqdm(test_data, desc=f"{subject:40s}", leave=False):
-        prompt = build_prompt(item["question"], item, subject, n_shot, dev_data or [])
-        context_ids = tokenizer.encode(prompt)
+        raw_prompt = build_prompt(
+            item["question"], item, subject, n_shot, dev_data or []
+        )
+        context = apply_chat(tokenizer, raw_prompt, n_shot, dev_data or [])
+        context_ids = tokenizer.encode(context)
         scores = {
             c: choice_logprob(model, tokenizer, context_ids, c, device)
             for c in ("A", "B", "C", "D")