text: Title: NameError: name 'Nmf' is not defined
  Body: #### Problem description
  
  Trying to run NMF using gensim, but it doesn't work.
  
  #### Steps/code/corpus to reproduce
  
  I tried to run what documentation suggests:
  
  ```python
  from gensim.test.utils import common_texts
  from gensim.corpora.dictionary import Dictionary
  
  # Create a corpus from a list of texts
  common_dictionary = Dictionary(common_texts)
  common_corpus = [common_dictionary.doc2bow(text) for text in common_texts]
  
  # Train the model on the corpus.
  nmf = Nmf(common_corpus, num_topics=10)
  ```
  but it throws the error in subject. 
  
  I tried to inspect the package and it seems it's not installed or doesn't exist. I don't understand why.
  
  #### Versions
  
  gensim 3.8.3
  
  