Flattens the hidden state from all LSTM layers into one tensor (for
        the sequence generator).

def package_hidden(self):
        """
        Flattens the hidden state from all LSTM layers into one tensor (for
        the sequence generator).
        """
        if self.inference:
            hidden = torch.cat(tuple(itertools.chain(*self.next_hidden)))
        else:
            hidden = None
        return hidden