Delete jac field instead of setting to None

Author: ValerianRey

src/torchjd/utils/_accumulation.py

@@ -6,7 +6,7 @@
 def _accumulate_jacs(params: Iterable[Tensor], jacobians: Iterable[Tensor]) -> None:
     for param, jac in zip(params, jacobians, strict=True):
         _check_expects_grad(param)
-        if hasattr(param, "jac") and param.jac is not None:
+        if hasattr(param, "jac"):
             param.jac += jac
         else:
             # TODO: this could be a serious memory issue

src/torchjd/utils/_jac_to_grad.py

@@ -66,10 +66,10 @@ def _disunite_gradient(
 
 def _free_jacs(params: Iterable[Tensor]) -> None:
     """
-    Clears the ``.jac`` fields of the provided parameters by setting them to ``None``.
+    Deletes the ``.jac`` field of the provided parameters.
 
     :param params: The parameters whose ``.jac`` fields should be cleared.
     """
 
     for p in params:
-        p.jac = None
+        del p.jac