code: def precompute_idfs(self, dfs, num_docs):
      return dict()
  
imports: from collections import defaultdict
  import math
  import unittest
  from gensim.models.bm25model import BM25ABC
  from gensim.models import OkapiBM25Model, LuceneBM25Model, AtireBM25Model
  from gensim.corpora import Dictionary
label_desc: Use image node_id 2 for calling the BM25Stub obj's underlying member method code with example usage: obj.precompute_idfs(dfs, num_docs) and returns: dict