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
