code: def merge_index(self):
      ivf_file = f"{self.index_file}.ivfdata"
  
      assert os.path.exists(self.index_template_file)
      assert not os.path.exists(
          ivf_file
      ), f"file with embeddings data {ivf_file} not found, check."
      assert not os.path.exists(self.index_file)
      index = faiss.read_index(self.index_template_file)
      block_fnames = [
          f"{self.index_shard_prefix}{i}" for i in range(self.nshards)
      ]
      for fn in block_fnames:
          assert os.path.exists(fn)
      logging.info(block_fnames)
      logging.info("merging...")
      merge_ondisk(index, block_fnames, ivf_file)
      logging.info("writing index...")
      faiss.write_index(index, self.index_file)
      logging.info("done")
  
imports: import faiss
  import numpy
  import os
  from tqdm import tqdm, trange
  import sys
  import logging
  from faiss.contrib.ondisk import merge_ondisk
  from faiss.contrib.big_batch_search import big_batch_search
  from faiss.contrib.exhaustive_search import knn_ground_truth
  from faiss.contrib.evaluation import knn_intersection_measure
  from utils import get_intersection_cardinality_frequencies, margin, is_pretransform_index
  from dataset import create_dataset_from_oivf_config
label_desc: Use image node_id 8 for calling the OfflineIVF obj's underlying member method code with example usage: obj.merge_index() without return types