Deprecate use of "default" and Variable as OpFromGrah overrides

tests/compile/test_builders.py

@@ -11,7 +11,7 @@ from pytensor.configdefaults import config
 from pytensor.gradient import DisconnectedType, Rop, disconnected_type, grad
 from pytensor.graph.basic import equal_computations
 from pytensor.graph.fg import FunctionGraph
-from pytensor.graph.null_type import NullType
+from pytensor.graph.null_type import NullType, null_type
 from pytensor.graph.rewriting.utils import rewrite_graph
 from pytensor.graph.utils import MissingInputError
 from pytensor.printing import debugprint
@@ -93,6 +93,20 @@ class TestOpFromGraph(unittest_tools.InferShapeTester):
         assert res.shape == (2, 5)
         assert np.all(180.0 == res)
 
+    def test_overrides_deprecated_api(self):
+        inp = scalar("x")
+        out = inp + 1
+        for kwarg in ("lop_overrides", "grad_overrides", "rop_overrides"):
+            with pytest.raises(
+                ValueError, match="'default' is no longer a valid value for overrides"
+            ):
+                OpFromGraph([inp], [out], **{kwarg: "default"})
+
+            with pytest.raises(
+                TypeError, match="Variables are no longer valid types for overrides"
+            ):
+                OpFromGraph([inp], [out], **{kwarg: null_type()})
+
     @pytest.mark.parametrize(
         "cls_ofg", [OpFromGraph, partial(OpFromGraph, inline=True)]
     )
@@ -211,9 +225,7 @@ class TestOpFromGraph(unittest_tools.InferShapeTester):
         w, b = vectors("wb")
         # we make the 3rd gradient default (no override)
         with pytest.warns(FutureWarning, match="grad_overrides is deprecated"):
-            op_linear = cls_ofg(
-                [x, w, b], [x * w + b], grad_overrides=[go1, go2, "default"]
-            )
+            op_linear = cls_ofg([x, w, b], [x * w + b], grad_overrides=[go1, go2, None])
         xx, ww, bb = vector("xx"), vector("yy"), vector("bb")
         zz = pt_sum(op_linear(xx, ww, bb))
         dx, dw, db = grad(zz, [xx, ww, bb])