    def get_output_dim(self) -> int:
        """
        Returns the dimension of the final vector output by this `Seq2VecEncoder`.  This is `not`
        the shape of the returned tensor, but the last element of that shape.
        """
        raise NotImplementedError
Returns the dimension of the final vector output by this `Seq2VecEncoder`.  This is `not`
the shape of the returned tensor, but the last element of that shape.
allennlp/modules/seq2vec_encoders/seq2vec_encoder.py