fix issue2321:Provide an implementation of the truncate method (#2357)

# Description


构建社区摘要时（community_store.py中的build_communities方法），先清空向量存储，仅提供了ChromaStore对于实现truncate方法，未提供MilvusStore对于该方法的实现，当使用milvus作为向量数据库时会抛出错误“NotImplementedError”。提供一种实现方法，修复此问题。

# How Has This Been Tested?

使用milvus作为向量数据库，然后通过以下步骤：
1.构建一个知识图谱类型的知识库；
2.上传一个文件；
3.选择上传的文件，点击"同步"

# Snapshots:

Include snapshots for easier review.

# Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have already rebased the commits and make the commit message
conform to the project standard.
- [x] I have performed a self-review of my own code
- [x] I have commented my code, particularly in hard-to-understand areas
- [ ] I have made corresponding changes to the documentation
- [ ] Any dependent changes have been merged and published in downstream
modules

packages/dbgpt-ext/src/dbgpt_ext/storage/vector_store/milvus_store.py

@@ -639,3 +639,15 @@ class MilvusStore(VectorStoreBase):
         else:
             metadata_filter_expr = metadata_filters[0]
         return metadata_filter_expr
+
+    def truncate(self):
+        """Truncate milvus collection.
+        """
+        logger.info(f"begin truncate milvus collection:{self.collection_name}")
+        from pymilvus import utility
+        if utility.has_collection(self.collection_name):
+            utility.drop_collection(self.collection_name)
+
+        logger.info(
+            f"truncate milvus collection {self.collection_name} success"
+        )