fix(mgb): register invalid grad for AddUpdate

GitOrigin-RevId: f9bbf570dcdc2aca714f3d9f3b7831c82bc7c3a9

src/opr/impl/basic_arith.cpp

@@ -947,6 +947,11 @@ void AddUpdate::record_execute_deps(ExecDependencyArray& deps) {
     record_megdnn_opr(deps);
 }
 
+MGB_IMPL_OPR_GRAD(AddUpdate) {
+    // actually valid, just not implemented
+    return InvalidGrad::make(opr, wrt_idx);
+}
+
 /* =========================== Reduce =========================== */
 
 class Reduce::KernScheduler {

src/opr/test/basic_arith/others.cpp

@@ -372,7 +372,7 @@ TEST(TestOprBasicArith, AddUpdateVolatile) {
             for (size_t i = 0; i < SIZE * 2; i ++) {
                 MGB_ASSERT_FLOAT_EQ(expect(i), z[i]);
             }
-            mgb_assert(host_sub.shape().total_nr_elems() == 4 && 
+            mgb_assert(host_sub.shape().total_nr_elems() == 4 &&
                 host_sub.layout().is_contiguous());
             for (size_t i = 0; i < 4; ++ i) {
                 size_t idx = i * (SIZE >> 1);
@@ -390,6 +390,27 @@ TEST(TestOprBasicArith, AddUpdateVolatile) {
     }
 }
 
+// AddUpdate in gradient path but no gradient flows through it
+TEST(TestOprBasicArith, AddUpdateInGradPath) {
+    auto graph = ComputingGraph::make();
+    HostTensorGenerator<> gen;
+    auto dest = opr::SharedDeviceTensor::make(*graph, *gen({42}));
+    auto host_x = gen({42});
+    auto x = opr::Host2DeviceCopy::make(*graph, host_x);
+    // delta depends on x, but not differentiable wrt x
+    // a invalid grad is registered for AddUpdate to fix this case
+    auto delta = opr::VirtualDep::make({opr::SetGrad::make(x, nullptr), x});
+    auto updated = opr::AddUpdate::make(dest, delta);
+    auto y = opr::reduce_ax_sum(updated + x, 0);
+    auto dx = cg::grad(y, x);
+    HostTensorND host_dx;
+    auto func = graph->compile({make_callback_copy(dx, host_dx)});
+    func->execute();
+    for (size_t i = 0; i < host_dx.shape(0); ++i) {
+        MGB_ASSERT_FLOAT_EQ(host_dx.ptr<float>()[i], 1.f);
+    }
+}
+
 TEST(TestOprBasicArith, MemFwd) {
     constexpr size_t SIZE = 12321;
     HostTensorGenerator<> gen;