    def extract_features(
        self, prev_output_tokens, encoder_out=None, incremental_state=None, **kwargs
    ):
        """
        Returns:
            tuple:
                - the decoder's features of shape `(batch, tgt_len, embed_dim)`
                - a dictionary with any model-specific outputs
        """
        raise NotImplementedError
Returns:
tuple:
- the decoder's features of shape `(batch, tgt_len, embed_dim)`
- a dictionary with any model-specific outputs
fairseq/models/fairseq_incremental_decoder.py