Forward computation.

        Parameters
        ----------
        data_batch : DataBatch
        is_train : bool
            Defaults to ``None``, in which case `is_train` is take as ``self.for_training``.

def forward(self, data_batch, is_train=None):
        """Forward computation.

        Parameters
        ----------
        data_batch : DataBatch
        is_train : bool
            Defaults to ``None``, in which case `is_train` is take as ``self.for_training``.
        """
        assert self.binded and self.params_initialized
        self.switch_bucket(data_batch.bucket_key, data_batch.provide_data,
                           data_batch.provide_label)
        self._curr_module.forward(data_batch, is_train=is_train)