Add CubeS₂ 4th-order spline support for charge assignment in SOG

- Introduced new method `precompute_cubes2_influence()` in SOGKSpace to handle CubeS₂ influence functions.
- Added a new header file `sog_spline.h` containing definitions and weight functions for CubeS₂ 4th-order splines.
- Updated SOGKSpace class to include new member variables for CubeS₂ influence functions and spline type.
- The CubeS₂ spline implementation is based on the reference: Predescu, Bergdorf, Shaw, J. Chem. Phys. 153, 224117 (2020).

Author: Johnsonjz

deepmd/pt/train/training.py

@@ -1560,7 +1560,8 @@ def save_model(self, save_path: str, lr: float = 0.0, step: int = 0) -> None:
         ]
         if len(checkpoint_files) > self.max_ckpt_keep:
             checkpoint_files.sort(key=lambda x: x.stat().st_mtime)
-            checkpoint_files[0].unlink()
+            for f in checkpoint_files[: len(checkpoint_files) - self.max_ckpt_keep]:
+                f.unlink()
 
     def get_data(
         self, is_train: bool = True, task_key: str = "Default"