Fixed small problem in frontend when using python 3.6

hpvm/projects/torch2hpvm/torch2hpvm/compile.py

@@ -201,7 +201,8 @@ class ModelExporter:
 
         def link_from_to(from_: PathLike, to: PathLike):
             from_, to = Path(from_), Path(to)
-            from_.unlink(missing_ok=True)
+            if from_.exists():
+                from_.unlink()
             from_.symlink_to(to.absolute())
 
         if isinstance(dataset, BinDataset):