def METHOD_NAME(self, xs, ilens, ys, olens):
    """Compute loss for training.
    :param xs: batch of padded source sequences paddle.Tensor (B, Tmax, idim)
    :param ilens: batch of lengths of source sequences (B), paddle.Tensor
    :param ys: batch of padded target sequences paddle.Tensor (B, Lmax)
    :param olens: batch of lengths of target sequences (B), paddle.Tensor
    :return: loss value
    :rtype: paddle.Tensor
    """
    raise NotImplementedError("forward method is not implemented")