code: def _encode_sequence(self, inputs, token_types, valid_length=None):
          
          # embedding
          word_embedding = self.word_embed(inputs)
          type_embedding = self.token_type_embed(token_types)
          embedding = word_embedding + type_embedding
          # encoding
          outputs, additional_outputs = self.encoder(embedding, None, valid_length)
          return outputs, additional_outputs
docs: Generate the representation given the input sequences.
  
          This is used for pre-training or fine-tuning a BERT model.