class_docstr: Loads a pre-weighted inverted index of token/document terms.
  
  Scores new queries by taking sparse dot products.
docstr: Convert doc_index --> doc_id.
code: def get_doc_id(self, doc_index):
      """
      Convert doc_index --> doc_id.
      """
      return self.doc_dict[1][doc_index] if self.doc_dict else doc_index
  
file_path: ParlAI/parlai/agents/tfidf_retriever/tfidf_doc_ranker.py
imports: import numpy
  import scipy.sparse
  from .None import utils
  from .None import tokenizers
  from parlai.utils.logging import logger
label_desc: Use image node_id 2 for calling the TfidfDocRanker obj's underlying member method code with example usage: obj.get_doc_id(doc_index) and returns: unknown