<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def maybe_build_from_corpus(self, corpus_generator, **kwargs):
 """Call SubwordTextEncoder.build_from_corpus is encoder_cls is such."""

    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)