Add top_k for create_chat and update_chat api (#4294)

### What problem does this PR solve?

Add top_k for create_chat and update_chat api #4157

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

---------

Co-authored-by: liuhua <10215101452@stu.ecun.edu.cn>

sdk/python/ragflow_sdk/modules/chat.py

@@ -28,6 +28,7 @@ class Chat(Base):
             self.similarity_threshold = 0.2
             self.keywords_similarity_weight = 0.7
             self.top_n = 8
+            self.top_k = 1024
             self.variables = [{"key": "knowledge", "optional": True}]
             self.rerank_model = None
             self.empty_response = None