code: 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]
docs: 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``.