title: Convert HuggingFace slow tokenizer to a fast tokenizer
body: <p>I have a model trained to disk with a slow tokenizer:</p>
  <pre><code>from transformers import convert_slow_tokenizer
  from transformers import BertTokenizer, BertForSequenceClassificationa
  
  mybert = BertForSequenceClassification.from_pretrained(PATH,
                                                          local_files_only=True,
                                                          )
  tokenizer = BertTokenizer.from_pretrained(PATH, 
                                            local_files_only=True, 
                                            use_fast=True)
  </code></pre>
  <p>I am able to use it to tokenize like so:</p>
  <pre><code>tokenized_example = tokenizer(
      mytext,
      max_length=100,
      truncation=&quot;only_second&quot;,
      return_overflowing_tokens=True,
      stride=50
  )
  </code></pre>
  <p>However, it is non-fast:</p>
  <pre><code>tokenized_example.is_fast
  False
  </code></pre>
  <p>I try to convert it to fast one, which looks successful</p>
  <pre><code>tokenizer = convert_slow_tokenizer.convert_slow_tokenizer(tokenizer)
  </code></pre>
  <p>However, now running this gives me:</p>
  <pre><code>tokenized_example = tokenizer(
      mytext,
      max_length=100,
      truncation=&quot;only_second&quot;,
      return_overflowing_tokens=True,
      stride=50
  )
  
  
  TypeError: 'tokenizers.Tokenizer' object is not callable
  </code></pre>
  <p>How can I convert this slow tokenizer to a fast one?</p>
  <p>I have <a href="https://stackoverflow.com/questions/65431837/transformers-v4-x-convert-slow-tokenizer-to-fast-tokenizer">seen this answer</a> and I have sentencepiece installed---this did not fix my issue.</p>