    def get_output_dim(self) -> int:
        """
        Returns the dimension of each vector in the sequence output by this `Seq2SeqEncoder`.
        This is `not` the shape of the returned tensor, but the last element of that shape.
        """
        raise NotImplementedError
Returns the dimension of each vector in the sequence output by this `Seq2SeqEncoder`.
This is `not` the shape of the returned tensor, but the last element of that shape.
allennlp/modules/seq2seq_encoders/seq2seq_encoder.py