[Relay] Add a non-recursive LetNode VisitExpr_ for LabelOps Pass to avoid stack overflow (apache#8917)

* Add a non-recursive Let VisitExpr_ for LabelOps

* fake commit to retrigger CI

* fake commit to retrigger the CI

* fix CI issue

* fix CI issue

Author: FrankYzy

src/relay/transforms/label_ops.cc

@@ -77,6 +77,25 @@ class LabelOpsMutator : public MixedModeMutator {
     }
     return std::move(f);
   }
+  Expr VisitExpr_(const LetNode* op) final {
+    auto pre_visit = [this](const LetNode* op) {
+      this->Mutate(op->var);
+      this->Mutate(op->value);
+    };
+    auto post_visit = [this](const LetNode* op) {
+      Var var = Downcast<Var>(this->Mutate(op->var));
+      auto value = this->Mutate(op->value);
+      auto body = this->Mutate(op->body);
+      auto expr = GetRef<Expr>(op);
+      if (var.same_as(op->var) && value.same_as(op->value) && body.same_as(op->body)) {
+        this->memo_[expr] = expr;
+      } else {
+        this->memo_[expr] = Let(var, value, body);
+      }
+    };
+    ExpandANormalForm(op, pre_visit, post_visit);
+    return memo_[GetRef<Expr>(op)];
+  }
 
   Expr Rewrite_(const CallNode* op, const Expr& post) final {
     auto updated = MixedModeMutator::Rewrite_(op, post);