    def forward(self, src_tokens, src_lengths=None, **kwargs):
        """
        Args:
            src_tokens (LongTensor): tokens in the source language of shape
                `(batch, src_len)`
            src_lengths (LongTensor): lengths of each source sentence of shape
                `(batch)`
        """
        raise NotImplementedError
Args:
src_tokens (LongTensor): tokens in the source language of shape
`(batch, src_len)`
src_lengths (LongTensor): lengths of each source sentence of shape
`(batch)`
fairseq/models/fairseq_encoder.py