change ckpt_prefix to relative path

Before, `ckpt_prefix` is an absolute path and the generated symilnks link to an abosulote path. If users wants to move the whole directory, the relative path should be more user-friendly.

Signed-off-by: Jinzhe Zeng <jinzhe.zeng@rutgers.edu>

deepmd/train/trainer.py

@@ -811,7 +811,7 @@ def save_checkpoint(self, cur_batch: int):
         try:
             ckpt_prefix = self.saver.save(
                 self.sess,
-                os.path.join(os.getcwd(), self.save_ckpt),
+                self.save_ckpt,
                 global_step=cur_batch,
             )
         except google.protobuf.message.DecodeError as e: