Revert "removed get_knowledge_graph"

lightrag/kg/postgres_impl.py

@@ -7,6 +7,7 @@ from typing import Any, Union, final
 import numpy as np
 import configparser
 
+from lightrag.types import KnowledgeGraph
 
 import sys
 from tenacity import (
@@ -1084,6 +1085,11 @@ class PGGraphStorage(BaseGraphStorage):
     ) -> tuple[np.ndarray[Any, Any], list[str]]:
         raise NotImplementedError
 
+    async def get_knowledge_graph(
+        self, node_label: str, max_depth: int = 5
+    ) -> KnowledgeGraph:
+        raise NotImplementedError
+
     async def drop(self) -> None:
         """Drop the storage"""
         drop_sql = SQL_TEMPLATES["drop_vdb_entity"]