'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):

    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]
