Forward computation. Here we do nothing but to keep a reference to
        the scores and the labels so that we can do backward computation.

        Parameters
        ----------
        data_batch : DataBatch
            Could be anything with similar API implemented.
        is_train : bool
            Default is ``None``, which means `is_train` takes the value of ``self.for_training``.

def forward(self, data_batch, is_train=None):
        """Forward computation. Here we do nothing but to keep a reference to
        the scores and the labels so that we can do backward computation.

        Parameters
        ----------
        data_batch : DataBatch
            Could be anything with similar API implemented.
        is_train : bool
            Default is ``None``, which means `is_train` takes the value of ``self.for_training``.
        """
        self._scores = data_batch.data[0]

        if is_train is None:
            is_train = self.for_training

        if is_train:
            self._labels = data_batch.label[0]