def METHOD_NAME(self, batch, batch_nb):
    """Need to overwrite PL validation_step to do validation.
    Args:
        batch: the object returned by the loader (a list of torch.Tensor
            in most cases) but can be something else.
        batch_nb (int): The number of the batch in the epoch.
    """
    loss = self.common_step(batch, batch_nb, train=False)
    self.log("val_loss", loss, on_epoch=True, prog_bar=True)