Refa: Remove useless conver and fix a bug for DefaultRerank (#8887)

### What problem does this PR solve?

1. bug when re-try, we need to reset i.
2. remove useless convert

### Type of change

- [x] Refactoring

rag/llm/rerank_model.py

         old_dynamic_batch_size = self._dynamic_batch_size
         if max_batch_size is not None:
             self._dynamic_batch_size = max_batch_size
-        res = []
+        res = np.array([], dtype=float)
         i = 0
         while i < len(pairs):
+            cur_i = i
             current_batch = self._dynamic_batch_size
             max_retries = 5
             retry_count = 0
                 try:
                     # call subclass implemented batch processing calculation
                     batch_scores = self._compute_batch_scores(pairs[i : i + current_batch])
-                    res.extend(batch_scores)
+                    res = np.append(res, batch_scores)
                     i += current_batch
                     self._dynamic_batch_size = min(self._dynamic_batch_size * 2, 8)
                     break
                     if "CUDA out of memory" in str(e) and current_batch > self._min_batch_size:
                         current_batch = max(current_batch // 2, self._min_batch_size)
                         self.torch_empty_cache()
+                        i = cur_i # reset i to the start of the current batch
                         retry_count += 1
                     else:
                         raise
             scores = self._model.compute_score(batch_pairs)
         else:
             scores = self._model.compute_score(batch_pairs, max_length=max_length)
-        scores = sigmoid(np.array(scores)).tolist()
+        scores = sigmoid(np.array(scores))
         if not isinstance(scores, Iterable):
             scores = [scores]
         return scores