symlink model.ckpt.* to relative paths (deepmodeling#2720)

Before, `model.ckpt.*` is symilnks to an abosulote path. If users wants
to move the whole directory, the relative path will be more
user-friendly.

---------

Signed-off-by: Jinzhe Zeng <[email protected]>

Author: njzjz

deepmd/train/trainer.py

@@ -832,7 +832,7 @@ def save_checkpoint(self, cur_batch: int):
                 pass
             if platform.system() != "Windows":
                 # by default one does not have access to create symlink on Windows
-                os.symlink(ori_ff, new_ff)
+                os.symlink(os.path.relpath(ori_ff, os.path.dirname(new_ff)), new_ff)
             else:
                 shutil.copyfile(ori_ff, new_ff)
         log.info("saved checkpoint %s" % self.save_ckpt)