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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _tokens_to_subtoken_ids(self, tokens):
 """Converts a list of tokens to a list of subtoken ids. Args: tokens: a list of strings. Returns: a list of integers in the range [0, vocab_size) """

    ret = []
    for token in tokens:
      ret.extend(self._token_to_subtoken_ids(token))
    return ret