Fetch chunk by batches. (#4177)

### What problem does this PR solve?

#4173

### Type of change

- [x] Performance Improvement

rag/nlp/search.py

         pg = int(req.get("page", 1)) - 1
         topk = int(req.get("topk", 1024))
         ps = int(req.get("size", topk))
-        offset, limit = pg * ps, (pg + 1) * ps
+        offset, limit = pg * ps, ps
 
         src = req.get("fields", ["docnm_kwd", "content_ltks", "kb_id", "img_id", "title_tks", "important_kwd", "position_int",
                                  "doc_id", "page_num_int", "top_int", "create_timestamp_flt", "knowledge_graph_kwd", "question_kwd", "question_tks",
 
     def chunk_list(self, doc_id: str, tenant_id: str, kb_ids: list[str], max_count=1024, fields=["docnm_kwd", "content_with_weight", "img_id"]):
         condition = {"doc_id": doc_id}
-        res = self.dataStore.search(fields, [], condition, [], OrderByExpr(), 0, max_count, index_name(tenant_id), kb_ids)
-        dict_chunks = self.dataStore.getFields(res, fields)
-        return dict_chunks.values()
+        res = []
+        bs = 128
+        for p in range(0, max_count, bs):
+            res = self.dataStore.search(fields, [], condition, [], OrderByExpr(), p, bs, index_name(tenant_id), kb_ids)
+            dict_chunks = self.dataStore.getFields(res, fields)
+            if dict_chunks:
+                res.extend(dict_chunks.values())
+            if len(dict_chunks.values()) < bs:
+                break
+        return res

rag/utils/es_conn.py

             s = s.sort(*orders)
 
         if limit > 0:
-            s = s[offset:limit]
+            s = s[offset:offset+limit]
         q = s.to_dict()
         logger.debug(f"ESConnection.search {str(indexNames)} query: " + json.dumps(q))