code: def maybe_build_from_corpus(self, corpus_generator, **kwargs):
      
      if self._encoder_cls is not text_lib.SubwordTextEncoder:
        return
      if self.encoder:
        return
  
      vocab_size = self._encoder_config.vocab_size
      self.encoder = text_lib.SubwordTextEncoder.build_from_corpus(
          corpus_generator=corpus_generator,
          target_vocab_size=vocab_size,
          **kwargs)
docs: Call SubwordTextEncoder.build_from_corpus is encoder_cls is such.