fix(imperative): fix the segmentfault when reduce backward

GitOrigin-RevId: 8a3e63d4f5

imperative/python/src/grad_override.cpp

@@ -408,7 +408,7 @@ std::optional<ValueRefList> reduce_grad_rule(
             [shapes = std::move(input_shapes), axis, keepdim](Span<ValueRef> grads) {
                 mgb_assert(grads.size() == 1);
                 ValueRef grad = grads[0];
-                if (!keepdim) {
+                if (!keepdim && grad) {
                     auto&& grad_op = AddAxis::make(std::vector<int32_t>({axis}));
                     grad = imperative::apply(*grad_op, grad)[0];
                 }