refine rerank (#1056)

### What problem does this PR solve?


### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

rag/llm/rerank_model.py

         token_count = 0
         for _, t in pairs:
             token_count += num_tokens_from_string(t)
-        batch_size = 32
+        batch_size = 4096
         res = []
         for i in range(0, len(pairs), batch_size):
             scores = self._model.compute_score(pairs[i:i + batch_size], max_length=2048)
-            scores = sigmoid(np.array(scores)).tolist()
-            res.extend(scores)
+            if isinstance(scores, float): res.append(scores)
+            else:  res.extend(scores)
         return np.array(res), token_count
 
 
         for i in range(0, len(pairs), batch_size):
             scores = self._model.compute_score(pairs[i:i + batch_size], max_length=self._model.max_length)
             scores = sigmoid(np.array(scores)).tolist()
+            if isinstance(scores, float): res.append(scores)
             res.extend(scores)
         return np.array(res), token_count
     
 
+