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/model/automodel.py

@@ -60,10 +60,9 @@ class AutoModel(BaseFactory["AutoModel"], nn.Module):
         model_path = Path(path)
 
         # Load config
-        config = ModelConfig()
         config_path = model_path / "config.json"
         if config_path.exists():
-            config.load(str(config_path))
+            config = ModelConfig.from_file(str(config_path))
         else:
             raise FileNotFoundError(f"Config file not found: {config_path}")
 
@@ -89,7 +88,7 @@ class AutoModel(BaseFactory["AutoModel"], nn.Module):
         save_path.mkdir(parents=True, exist_ok=True)
 
         # Save config
-        self.config.save(str(save_path / "config.json"))
+        self.config.to_file(str(save_path / "config.json"))
 
         # Save weights
         st.save_file(self.state_dict(), str(save_path / "model.safetensors"))