examples/legacy/pytorch-lightning/lightning_base.py/LoggingCallback/on_validation_end
class LoggingCallback:    def on_validation_end(self, trainer: pl.Trainer, pl_module: pl.LightningModule):
        rank_zero_info("***** Validation results *****")
        metrics = trainer.callback_metrics
        # Log results
        for key in sorted(metrics):
            if key not in ["log", "progress_bar"]:
                rank_zero_info("{} = {}\n".format(key, str(metrics[key])))