title: Transformers v4.x: Convert slow tokenizer to fast tokenizer
body: <p>I'm following the transformer's pretrained model <a href="https://huggingface.co/joeddav/xlm-roberta-large-xnli?text=%0A&amp;candidate_labels=&amp;multi_class=true" rel="noreferrer">xlm-roberta-large-xnli</a> example</p>
  <pre><code>from transformers import pipeline
  classifier = pipeline(&quot;zero-shot-classification&quot;,
                        model=&quot;joeddav/xlm-roberta-large-xnli&quot;)
  </code></pre>
  <p>and I get the following error</p>
  <pre><code>ValueError: Couldn't instantiate the backend tokenizer from one of: (1) a `tokenizers` library serialization file, (2) a slow tokenizer instance to convert or (3) an equivalent slow tokenizer class to instantiate and convert. You need to have sentencepiece installed to convert a slow tokenizer to a fast one.
  </code></pre>
  <p>I'm using Transformers version <code>'4.1.1'</code></p>