docstr: Saves the index to the cache directory.
code: def save_index(self, name: str, cache_directory: str):
      """Saves the index to the cache directory."""
      pass
  
imports: import hashlib
  import json
  import os
  from abc import ABC, abstractmethod
  from typing import Any, Callable, Dict, List, Optional, Type, TYPE_CHECKING, Union
  import numpy
  import pandas
  from tqdm import tqdm
  from ludwig.vector_index import FAISS, get_vector_index_cls
  from ludwig.vector_index.base import VectorIndex
  from ludwig.utils.batch_size_tuner import BatchSizeEvaluator
  from ludwig.utils.torch_utils import get_torch_device
label_desc: Use image node_id 3 for calling the RetrievalModel obj's underlying member method code with example usage: obj.save_index(name, cache_directory) without return types