[Ehn] Enhance config module

[HydrogenSulfate]

PR types

New features

PR changes

APIs

Describe

1. 通过ConfigStore，支持在yaml文件中指定defaults字段来提供通用参数的默认配置，免去重复的yaml字段填写和Solver实例化时的十多个参数指定，如下所示。

   # before solver = ppsci.solver.Solver( model, constraint, cfg.output_dir, optimizer, lr_scheduler, cfg.TRAIN.epochs, cfg.TRAIN.iters_per_epoch, save_freq=cfg.TRAIN.save_freq, log_freq=cfg.log_freq, eval_during_train=True, eval_freq=cfg.TRAIN.eval_freq, seed=cfg.seed, equation=equation, geom=geom, validator=validator, visualizer=visualizer, pretrained_model_path=cfg.TRAIN.pretrained_model_path, checkpoint_path=cfg.TRAIN.checkpoint_path, eval_with_no_grad=cfg.EVAL.eval_with_no_grad, )

   # after solver = ppsci.solver.Solver( model, constraint, optimizer=optimizer, equation=equation, validator=validator, visualizer=visualizer, cfg=cfg, <-- Only need cfg )

2. 移除原有的AttrDict相关代码。
3. 更新相关单测

此PR只负责更新基础功能，案例的代码、文档更新在 #852

[paddle-bot]

Thanks for your contribution!

[zhiminzhang0830]

LGTM