code: def get_dummy_canonical_hf_index_retriever(self):
      dataset = self.get_dummy_dataset()
      config = RagConfig(
          retrieval_vector_size=self.retrieval_vector_size,
          question_encoder=DPRConfig().to_dict(),
          generator=BartConfig().to_dict(),
      )
      with patch(
          "transformers.models.rag.retrieval_rag.load_dataset"
      ) as mock_load_dataset:
          mock_load_dataset.return_value = dataset
          retriever = RagRetriever(
              config,
              question_encoder_tokenizer=self.get_dpr_tokenizer(),
              generator_tokenizer=self.get_bart_tokenizer(),
          )
      return retriever
  
imports: import json
  import os
  import shutil
  import tempfile
  from unittest import TestCase
  from unittest.mock import patch
  import numpy
  from datasets import Dataset
  from transformers import is_faiss_available
  from transformers.models.bart.configuration_bart import BartConfig
  from transformers.models.bart.tokenization_bart import BartTokenizer
  from transformers.models.bert.tokenization_bert import VOCAB_FILES_NAMES
  from transformers.models.dpr.configuration_dpr import DPRConfig
  from transformers.models.dpr.tokenization_dpr import DPRContextEncoderTokenizer, DPRQuestionEncoderTokenizer
  from transformers.models.rag.configuration_rag import RagConfig
  from transformers.models.rag.retrieval_rag import CustomHFIndex, RagRetriever
  from transformers.models.roberta.tokenization_roberta import VOCAB_FILES_NAMES
  from transformers.testing_utils import require_faiss, require_sentencepiece, require_tokenizers, require_torch
label_desc: Use image node_id 7 for calling the RagRetrieverTest obj's underlying member method code with example usage: obj.get_dummy_canonical_hf_index_retriever() and returns: retriever