code: def persist(self):
      assert (
          self._index is not None
      ), "Please create an index before calling persist."
      import faiss
  
      faiss.write_index(self._index, self._index_path)
  
file_path: evadb/evadb/third_party/vector_stores/faiss.py
imports: import os
  from pathlib import Path
  from typing import List
  import numpy
  from evadb.third_party.vector_stores.types import FeaturePayload, VectorIndexQuery, VectorIndexQueryResult, VectorStore
  from evadb.utils.generic_utils import try_to_import_faiss
label_desc: Use image node_id 4 for calling the FaissVectorStore obj's underlying member method code with example usage: obj.persist() without return types