    def extract_features(
        self,
        prev_output_tokens,
        encoder_out=None,
        early_exit=None,
        layers=None,
        **unused
    ):
        """
        Similar to *forward* but only return features.
        Inputs:
            prev_output_tokens: Tensor(B, T)
            encoder_out: a dictionary of hidden states and masks

        Returns:
            tuple:
                - the decoder's features of shape `(batch, tgt_len, embed_dim)`
                - a dictionary with any model-specific outputs
            the LevenshteinTransformer decoder has full-attention to all generated tokens
        """
        # embed positions
        positions = (
            self.embed_positions(prev_output_tokens)
            if self.embed_positions is not None
            else None
        )

        # embed tokens and positions
        x = self.embed_scale * self.embed_tokens(prev_output_tokens)
        if self.project_in_dim is not None:
            x = self.project_in_dim(x)

        if positions is not None:
            x += positions
        x = self.dropout_module(x)

        # B x T x C -> T x B x C
        x = x.transpose(0, 1)
        attn = None
        inner_states = [x]

        # decoder layers
        decoder_padding_mask = prev_output_tokens.eq(self.padding_idx)
        layers = self.layers if layers is None else layers
        early_exit = len(layers) if early_exit is None else early_exit
        for _, layer in enumerate(layers[:early_exit]):
            x, attn, _ = layer(
                x,
                encoder_out["encoder_out"][0]
                if (encoder_out is not None and len(encoder_out["encoder_out"]) > 0)
                else None,
                encoder_out["encoder_padding_mask"][0]
                if (
                    encoder_out is not None
                    and len(encoder_out["encoder_padding_mask"]) > 0
                )
                else None,
                self_attn_mask=None,
                self_attn_padding_mask=decoder_padding_mask,
            )
            inner_states.append(x)

        if self.layer_norm:
            x = self.layer_norm(x)

        # T x B x C -> B x T x C
        x = x.transpose(0, 1)

        if self.project_out_dim is not None:
            x = self.project_out_dim(x)

        return x, {"attn": attn, "inner_states": inner_states}
Similar to *forward* but only return features.
Inputs:
prev_output_tokens: Tensor(B, T)
encoder_out: a dictionary of hidden states and masks

Returns:
tuple:
- the decoder's features of shape `(batch, tgt_len, embed_dim)`
- a dictionary with any model-specific outputs
the LevenshteinTransformer decoder has full-attention to all generated tokens
fairseq/models/nat/levenshtein_transformer.py