Change subfunction signature to ignore value

Instead of marking `value` for deletion by doing
```py
del value  # Unused.
```

ignore the variable in the function signature.

model.py

@@ -90,9 +90,7 @@ def ffn_size(emb_size, widening_factor):
 
 
 def apply_rules(rules):
-    def _apply_rules(path, value):
-        del value  # Unused.
-
+    def _apply_rules(path, _):
         path_list = [str(i.key).split("/") for i in path if isinstance(i, jax.tree_util.DictKey)]
         flattened_path = jax.tree_util.tree_flatten(path_list)[0]