fix(mgb/opr-mm): fix grad func of reduce and gather

GitOrigin-RevId: 4687faef9943460011737315cf052cc584e6988a

src/opr-mm/impl/collective_comm.cpp

@@ -139,7 +139,10 @@ public:
     VarNode* full_grad(VarNode* out_grad, const CollectiveComm* opr) const {
         auto mode = ModeTrait::from_mode(opr->param().mode).grad_mode();
         SymbolVarArray og_syms;
-        og_syms.push_back(out_grad);
+
+        if (out_grad != nullptr) {
+            og_syms.push_back(out_grad);
+        }
 
         auto&& cn = opr->output(0)->comp_node();
 
@@ -401,6 +404,11 @@ class CollectiveComm::ModeTrait::ReduceBase : public ReducedBasedTrait,
 class CollectiveComm::ModeTrait::REDUCE_SUM final : public ReduceBase {
     MegRay::ReduceOp op() const override { return MegRay::ReduceOp::MEGRAY_SUM; }
 
+    VarNode* grad(VarNode* out_grad, const CollectiveComm* opr) const override {
+        VarNode* input = opr->is_root() ? out_grad : nullptr;
+        return full_grad(input, opr);
+    }
+
     Mode grad_mode() override { return Mode::BROADCAST; }
 };
 
@@ -484,6 +492,11 @@ class CollectiveComm::ModeTrait::GATHER : public ModeTrait {
         mgb_assert(status == MegRay::MEGRAY_OK, "MegRay gather failed");
     }
 
+    VarNode* grad(VarNode* out_grad, const CollectiveComm* opr) const override {
+        VarNode* input = opr->is_root() ? out_grad : nullptr;
+        return full_grad(input, opr);
+    }
+
     Mode grad_mode() override { return Mode::SCATTER; }
 };