class_docstr: How2 Video Retriver.
  Reference usage of FAISS:
  https://github.com/fairinternal/fairseq-py/blob/paraphrase_pretraining/fairseq/data/multilingual_faiss_dataset.py
class_docstr_tok: ["How2","Video","Retriver",".","Reference","usage","of","FAISS",":","https",":","\/\/github.com\/fairinternal\/fairseq-py\/blob\/paraphrase_pretraining\/fairseq\/data\/multilingual_faiss_dataset.py"]
code: def save(self, out_dir):
      faiss.write_index(self.db, os.path.join(out_dir, "faiss_idx"))
      with open(
          os.path.join(out_dir, "videoid_to_vectoridx.pkl"), "wb"
      ) as fw:
          pickle.dump(
              self.videoid_to_vectoridx,
              fw,
              protocol=pickle.HIGHEST_PROTOCOL,
          )
  
imports: import os
  import numpy
  import pickle
  import time
  from collections import defaultdict
  from ..utils import get_local_rank, print_on_rank0
label_desc: Use image node_id 4 for calling the VectorRetriever obj's underlying member method code with example usage: obj.save(out_dir) without return types