fix: vectorstore pgvector ensure compatibility #3884 (#4248)

Ensure compatibility with both SQLAlchemy v1/v2 

fix the issue when using SQLAlchemy v1 (reported at #3884)

`
langchain/vectorstores/pgvector.py", line 168, in
create_tables_if_not_exists
    self._conn.commit()
AttributeError: 'Connection' object has no attribute 'commit'
`

Ref Doc :
https://docs.sqlalchemy.org/en/14/changelog/migration_20.html#migration-20-autocommit

langchain/vectorstores/pgvector.py

@@ -164,12 +164,12 @@ class PGVector(VectorStore):
             self.logger.exception(e)
 
     def create_tables_if_not_exists(self) -> None:
-        Base.metadata.create_all(self._conn)
-        self._conn.commit()
+        with self._conn.begin():
+            Base.metadata.create_all(self._conn)
 
     def drop_tables(self) -> None:
-        Base.metadata.drop_all(self._conn)
-        self._conn.commit()
+        with self._conn.begin():
+            Base.metadata.drop_all(self._conn)
 
     def create_collection(self) -> None:
         if self.pre_delete_collection: