Select model based on eval loss by default and display eval metric as well

[WenjieDu]

1. Feature description

Training models should be selected based on evaluation loss on the validation set. As for the eval metric, it should be applied and displayed for users' further information as well.

2. Motivation

The criterion for selecting the best model could be the higher the better, or the lower the better, making things complex. To unify and simplify this, just apply loss on the validation set for model selection. If eval metric is provided, calculate the value and display it for users as well.

3. Your contribution

Will make a PR