text: Title: Incorrect CBOW implementation in Gensim leads to inferior performance
  Body: #### Problem description
  
  According to this article https://aclanthology.org/2021.insights-1.1.pdf:
  
  <img width="636" alt="Screen Shot 2021-11-09 at 15 47 21" src="https://user-images.githubusercontent.com/610412/140945923-7d279468-a9e9-41b4-b7c2-919919832bc5.png">
  
  
  #### Steps/code/corpus to reproduce
  
  I haven't tried to verify / reproduce. Gensim's goal is to follow the original C implementation faithfully, which it does. So this is not a bug per se, more a question of "how whether / how much we want to deviate from the reference implementation". I'm in favour if the result is unambiguous better (more accurate, faster, no downsides).
  
  #### Versions
  
  All versions since the beginning of word2vec in Gensim.