    def _forward(self, src_tokens, src_lengths, return_all_hiddens=False):
        """
        Args:
            src_tokens: Input source tokens Tensor of shape B X T X C
            src_lengths: Lengths Tensor corresponding to input source tokens
            return_all_hiddens: If true will append the self attention states to the encoder states
        Returns:
            encoder_out: Tensor of shape B X T X C
            encoder_padding_mask: Optional Tensor with mask
            encoder_embedding: Optional Tensor. Always empty here
            encoder_states: List of Optional Tensors wih self attention states
            src_tokens: Optional Tensor. Always empty here
            src_lengths: Optional Tensor. Always empty here
        """
        x, input_lengths = self.subsample(src_tokens, src_lengths)  # returns T X B X C
        encoder_padding_mask = lengths_to_padding_mask(input_lengths)
        x = self.embed_scale * x
        if self.pos_enc_type == "rel_pos":
            positions = self.embed_positions(x)

        elif self.pos_enc_type == "rope":
            positions = None

        else:
            positions = self.embed_positions(encoder_padding_mask).transpose(0, 1)
            x += positions
            positions = None

        x = self.linear(x)
        x = self.dropout(x)
        encoder_states = []

        # x is T X B X C
        for layer in self.conformer_layers:
            x, _ = layer(x, encoder_padding_mask, positions)
            if return_all_hiddens:
                encoder_states.append(x)

        return {
            "encoder_out": [x],  # T x B x C
            "encoder_padding_mask": [encoder_padding_mask]
            if encoder_padding_mask.any()
            else [],  # B x T
            "encoder_embedding": [],  # B x T x C
            "encoder_states": encoder_states,  # List[T x B x C]
            "src_tokens": [],
            "src_lengths": [],
        }
Args:
src_tokens: Input source tokens Tensor of shape B X T X C
src_lengths: Lengths Tensor corresponding to input source tokens
return_all_hiddens: If true will append the self attention states to the encoder states
Returns:
encoder_out: Tensor of shape B X T X C
encoder_padding_mask: Optional Tensor with mask
encoder_embedding: Optional Tensor. Always empty here
encoder_states: List of Optional Tensors wih self attention states
src_tokens: Optional Tensor. Always empty here
src_lengths: Optional Tensor. Always empty here
fairseq/models/speech_to_text/s2t_conformer.py