class_docstr: Combo Faiss Retrieval Flask resource.
  The PUT method is to get KNN tokens, add new chunks, reset index.
docstr: write the dynamic index into a file
  Args:
      index_name: str, index name
docstr_tok: ["write","the","dynamic","index","into","a","file","Args",":","index_name",":","str",",","index","name"]
code: def write_index(self, index_name: str):
      """
      write the dynamic index into a file
      Args:
          index_name: str, index name
      """
      output = "success"
      if not self.updatable:
          if not self.updatable:
              return "no dynamic service, no action is performed"
      for i, service in enumerate(self.retrieval_services):
          if service.updatable:
              service.write_index(index_name)
      return output
  
imports: import pickle
  import threading
  from typing import List, Union
  import faiss
  import numpy
  import torch
  from flask import Flask, jsonify, request
  from flask_restful import Api, Resource
  from nemo.collections.common.tokenizers.tokenizer_spec import TokenizerSpec
  from nemo.collections.nlp.modules.common.megatron.retrieval_services.retrieval_service import DynamicFaissRetrievalService, FaissRetrievalService
  from nemo.collections.nlp.modules.common.megatron.retrieval_services.util import lock
label_desc: Use image node_id 7 for calling the ComboRetrievalResource obj's underlying member method code with example usage: obj.write_index(index_name) and returns: output, str