fix(mge): fix segfault with Function returning unused grads

GitOrigin-RevId: 0cce845923

imperative/python/src/grad.cpp

@@ -282,6 +282,10 @@ struct GradFn : std::enable_shared_from_this<GradFn> {
     }
 };
 
+GradSlotPtr::operator bool() const {
+    return bool(grad_fn);
+}
+
 GradSlot* GradSlotPtr::operator->() {
     return &grad_fn->slots[idx];
 }
@@ -537,7 +541,10 @@ void GradKey::backward(std::vector<TensorWrapper*> tensors, std::vector<TensorWr
         if (!grad_fn) continue;
 
         auto grad_receiver = [&](size_t i, auto&& g) {
-            accum_grad(grad_fn->dsts[i]->grad, std::forward<decltype(g)>(g));
+            auto& dst = grad_fn->dsts[i];
+            if (dst) {
+                accum_grad(dst->grad, std::forward<decltype(g)>(g));
+            }
         };
         std::visit([&](auto&& backward) {
             using T = std::decay_t<decltype(backward)>;

imperative/python/src/grad_info.h

@@ -22,6 +22,7 @@ struct GradSlotPtr {
     std::shared_ptr<GradFn> grad_fn;
     size_t idx;
 
+    operator bool() const;
     GradSlot* operator->();
 };