readme: ```python
  import tempfile
  
  from tokenizers import Tokenizer, models
  from transformers import PreTrainedTokenizerFast
  
  model_max_length = 4
  vocab = [(chr(i), i) for i in range(256)]
  tokenizer = Tokenizer(models.Unigram(vocab))
  with tempfile.NamedTemporaryFile() as f:
      tokenizer.save(f.name)
      real_tokenizer = PreTrainedTokenizerFast(tokenizer_file=f.name, model_max_length=model_max_length)
  
  real_tokenizer._tokenizer.save("dummy/tokenizer.json")
  
  ```
  
  config uses Albert which works with a minimal `config.json`