fix: saving ckpt error when ckpt folder has some folder inside (#3516)

Author: Crescent-Saturn

nerfstudio/engine/trainer.py

@@ -478,8 +478,8 @@ def save_checkpoint(self, step: int) -> None:
         )
         # possibly delete old checkpoints
         if self.config.save_only_latest_checkpoint:
-            # delete everything else in the checkpoint folder
-            for f in self.checkpoint_dir.glob("*"):
+            # delete every other checkpoint in the checkpoint folder
+            for f in self.checkpoint_dir.glob("*.ckpt"):
                 if f != ckpt_path:
                     f.unlink()