refactor: Config序列化统一BaseConfig基类

- 新增astrai/config/base.py，提供to_dict/from_dict基类
- 统一命名：load/save → from_file/to_file
- Checkpoint.meta合并训练配置到meta.json
- sys.stderr.warn → warnings.warn
- from_file改为classmethod

astrai/trainer/train_callback.py

@@ -125,6 +125,7 @@ class CheckpointCallback(TrainCallback):
             epoch=context.epoch,
             iteration=context.iteration,
             extra=extra,
+            meta=context.config.to_dict(),
         )
 
         context.checkpoint.save(save_path)