fix(grad): stop using exception in grad_override

GitOrigin-RevId: 00ae38d48b91e93f6de31d208de7ca469d79a938

imperative/python/src/grad.cpp

@@ -393,13 +393,11 @@ apply_result_t apply_grad(ApplyContext& ctx) {
         auto&& it = registry.find(ctx.op->dyn_typeinfo());
         if (it != registry.end()) {
             auto&& maker = grad_fn_holder.emplace<CustomBackward>().maker(ctx);
-            try {
-                auto ret = it->second(ctx, maker);
+            if (auto ret = it->second(ctx, maker)) {
                 maker.finalize();
-                return ret;
-            } catch (GradRuleFallback&) {
-                grad_fn_holder.reset();
+                return *ret;
             }
+            grad_fn_holder.reset();
         }
         return backward_graph_grad_rule(ctx, grad_fn_holder);
     }();

imperative/python/src/grad.h

@@ -16,6 +16,7 @@
 
 #include <megbrain/utils/small_vector.h>
 #include <memory>
+#include <optional>
 
 namespace mgb::imperative::python {
 
@@ -154,7 +155,7 @@ public:
     Maker maker(ApplyContext& ctx) {return {*this, ctx};}
 };
 
-using GradRuleFn = std::function<apply_result_t(ApplyContext&, CustomBackward::Maker&)>;
+using GradRuleFn = std::function<std::optional<apply_result_t>(ApplyContext&, CustomBackward::Maker&)>;
 
 std::unordered_map<Typeinfo*, GradRuleFn>& grad_rule_registry();
 

imperative/python/src/grad_override.cpp

@@ -37,14 +37,14 @@ std::shared_ptr<Tensor> broadcast_to(Tensor* x, Tensor* s) {
 
 std::shared_ptr<Tensor> make_empty_tensor(CompNode cn, Tensor* shape, DType dtype) {
     HostTensorND scalar{cn, {{1}, dtype}};
-    std:memset(scalar.raw_ptr(), 0, dtype.size());
+    std::memset(scalar.raw_ptr(), 0, dtype.size());
     interpreter::Interpreter::Handle handle = interpreter_for_py->put(scalar, false);
     auto&& t = std::make_shared<Tensor>(handle);
     auto res = broadcast_to(t.get(), shape);
     return res;
 }
 
-apply_result_t elemwise_grad_rule(ApplyContext& ctx, CustomBackward::Maker& maker) {
+std::optional<apply_result_t> elemwise_grad_rule(ApplyContext& ctx, CustomBackward::Maker& maker) {
     auto& op = ctx.op->cast_final_safe<Elemwise>();
     if (op.mode == Elemwise::Mode::ADD) {
         mgb_assert(ctx.nargs == 2);
@@ -71,10 +71,10 @@ apply_result_t elemwise_grad_rule(ApplyContext& ctx, CustomBackward::Maker& make
         });
         return apply(ctx);
     }
-    throw GradRuleFallback();
+    return {};
 }
 
-apply_result_t reshape_grad_rule(ApplyContext& ctx, CustomBackward::Maker& maker) {
+std::optional<apply_result_t> reshape_grad_rule(ApplyContext& ctx, CustomBackward::Maker& maker) {
     mgb_assert(ctx.nargs == 2);
     std::array<std::shared_ptr<Tensor>, 2> input_shapes;
     for (size_t i = 0; i < 2; ++i) {
@@ -100,7 +100,7 @@ apply_result_t reshape_grad_rule(ApplyContext& ctx, CustomBackward::Maker& maker
     return apply(ctx);
 }
 
-apply_result_t subtensor_grad_rule(ApplyContext& ctx, CustomBackward::Maker& maker) {
+std::optional<apply_result_t> subtensor_grad_rule(ApplyContext& ctx, CustomBackward::Maker& maker) {
     auto&& op = ctx.op->cast_final_safe<Subtensor>();
     auto&& grad_op = SetSubtensor::make(op.items);
     SmallVector<std::shared_ptr<Tensor>> inputs;
@@ -130,7 +130,7 @@ apply_result_t subtensor_grad_rule(ApplyContext& ctx, CustomBackward::Maker& mak
     return apply(ctx);
 }
 
-apply_result_t indexingMultiAxisVec_grad_rule(ApplyContext& ctx, CustomBackward::Maker& maker) {
+std::optional<apply_result_t> indexingMultiAxisVec_grad_rule(ApplyContext& ctx, CustomBackward::Maker& maker) {
     auto&& op = ctx.op->cast_final_safe<IndexingMultiAxisVec>();
     auto&& grad_op = IndexingSetMultiAxisVec::make(op.items);
     SmallVector<std::shared_ptr<Tensor>> inputs;
@@ -160,11 +160,11 @@ apply_result_t indexingMultiAxisVec_grad_rule(ApplyContext& ctx, CustomBackward:
     return apply(ctx);
 }
 
-apply_result_t reduce_grad_rule(ApplyContext& ctx, CustomBackward::Maker& maker) {
+std::optional<apply_result_t> reduce_grad_rule(ApplyContext& ctx, CustomBackward::Maker& maker) {
     auto& op = ctx.op->cast_final_safe<Reduce>();
     if (op.mode == Reduce::Mode::SUM) {
         if (ctx.nargs != 1) {
-            throw GradRuleFallback();
+            return {};
         }
         std::array<std::shared_ptr<Tensor>, 1> input_shapes;
         if (input_requires_grad(ctx, 0)) {
@@ -182,10 +182,10 @@ apply_result_t reduce_grad_rule(ApplyContext& ctx, CustomBackward::Maker& maker)
         });
         return apply(ctx);
     }
-    throw GradRuleFallback();
+    return {};
 }
 
-apply_result_t addAxis_grad_rule(ApplyContext& ctx, CustomBackward::Maker& maker) {
+std::optional<apply_result_t> addAxis_grad_rule(ApplyContext& ctx, CustomBackward::Maker& maker) {
     auto&& op = ctx.op->cast_final_safe<AddAxis>();
     mgb_assert(ctx.nargs == 1);
     bool flag = input_requires_grad(ctx, 0);
@@ -204,7 +204,7 @@ apply_result_t addAxis_grad_rule(ApplyContext& ctx, CustomBackward::Maker& maker
     return apply(ctx);
 }
 
-apply_result_t removeAxis_grad_rule(ApplyContext& ctx, CustomBackward::Maker& maker) {
+std::optional<apply_result_t> removeAxis_grad_rule(ApplyContext& ctx, CustomBackward::Maker& maker) {
     auto&& op = ctx.op->cast_final_safe<RemoveAxis>();
     mgb_assert(ctx.nargs == 1);
     bool flag = input_requires_grad(ctx, 0);
@@ -223,7 +223,7 @@ apply_result_t removeAxis_grad_rule(ApplyContext& ctx, CustomBackward::Maker& ma
     return apply(ctx);
 }
 
-apply_result_t fastpathcopy_grad_rule(ApplyContext& ctx, CustomBackward::Maker& maker) {
+std::optional<apply_result_t> fastpathcopy_grad_rule(ApplyContext& ctx, CustomBackward::Maker& maker) {
     mgb_assert(ctx.nargs == 1);
     maker.output_size(1).output_captured(0, false);
     maker.backward([](BackwardContext&, Tensor*const* grads, size_t ngrads) {