expand rerank range (#2746)

### What problem does this PR solve?


### Type of change

- [x] Performance Improvement

rag/nlp/search.py

@@ -364,7 +364,7 @@ def retrieval(self, question, embd_mdl, tenant_id, kb_ids, page, page_size, simi
         if not question:
             return ranks
         RERANK_PAGE_LIMIT = 3
-        req = {"kb_ids": kb_ids, "doc_ids": doc_ids, "size": page_size*RERANK_PAGE_LIMIT,
+        req = {"kb_ids": kb_ids, "doc_ids": doc_ids, "size": max(page_size*RERANK_PAGE_LIMIT, 128),
                "question": question, "vector": True, "topk": top,
                "similarity": similarity_threshold,
                "available_int": 1}