refactor: 统一序列化层，消除分散的 I/O 路径

- Checkpoint 改为 @dataclass，内聚 save/load 方法
- 提取 save_safetensors/load_safetensors/save_json/load_json 共享工具
- 新增 save_model/load_model_config/load_model_weights 模块函数
- automodel 和 lora 统一委托到 serialization 模块

tests/data/test_checkpoint.py

@@ -1,3 +1,4 @@
+import os
 import tempfile
 
 import torch
@@ -36,7 +37,6 @@ def test_single_process():
 
 
 def test_checkpoint_with_extra():
-    """Verify extra keys are saved as individual .pt files and loaded back."""
     model = torch.nn.Linear(10, 5)
     optimizer = AdamW(model.parameters(), lr=1e-3)
     optimizer.step()
@@ -52,8 +52,6 @@ def test_checkpoint_with_extra():
     with tempfile.TemporaryDirectory() as tmpdir:
         checkpoint.save(tmpdir)
 
-        import os
-
         assert os.path.exists(os.path.join(tmpdir, "optimizer.pt"))
         assert os.path.exists(os.path.join(tmpdir, "scheduler.pt"))