Fix validation loss

    def valid_step(self, sample):

        """Do forward pass in evaluation mode."""

        with torch.no_grad():

            self.model.eval()

        logging_output, sample_size = {}, 0

        with torch.no_grad():

            sample = self._prepare_sample(sample)

            if sample is None:

                sample = self._prepare_sample(self._dummy_batch)

                ignore_results = True

            else:

                ignore_results = False

            _loss, sample_size, logging_output = self.task.get_loss(

                self.model, self.criterion, sample,

            )

            if ignore_results:

                logging_output, sample_size = {}, 0

        # gather logging outputs from all replicas

        if self.args.distributed_world_size > 1:

            logging_output, sample_size = zip(*distributed_utils.all_gather_list(