[Change Request] Modify new `Evaluator` for the three tasks

[zhjwy9343]

The current GraphStorm Evaluators have misleading naming convention and lack of default metric. The new Evaluators will be task-specific.

Sample code

evaluator = gs.eval.GSgnnClassificationEvaluator(eval_frequency=100)

Requested changes

1. Create GSgnnClassificationEvaluator, setting "accuracy" as default evaluation metric.
2. Create GSgnnRegressionEvaluator, setting "mse" as default evaluation metric.
3. Create GSgnnLinkPredictionEvaluator, setting "mrr" as default evaluation metric.