def METHOD_NAME(self):
    """mask with B*T size for valid tokens in `payload`
    """
    if type(self._payload) is dict:
        B, T = next(v for k, v in self._payload.items() if self.is_seq_feature(k, v)).size()
    else:
        B, T = self._payload.size()[:2]
    return (torch.arange(T, device=self._length.device).unsqueeze(0).expand(B, T) < \
            self._length.unsqueeze(1)).long()