Change AOT from ExprVisitor to MixedModeVisitor (apache#8856)

This should allow better scale-ability for AOT when targeting larger networks.

src/relay/backend/aot_executor_codegen.cc

@@ -53,7 +53,7 @@ using StorageMap =
  * This is an on demand allocator for AOT. A new temporary
  * (storage allocator identifier) is allocated for each operation.
  */
-class AOTOnDemandAllocator : public ExprVisitor {
+class AOTOnDemandAllocator : public MixedModeVisitor {
  public:
   // run the visitor on a function.
   void Run(const Function& func) {
@@ -84,10 +84,7 @@ class AOTOnDemandAllocator : public ExprVisitor {
     AssignReturnSid(GetRef<Expr>(op));
   }
 
-  void VisitExpr_(const VarNode* op) final {
-    ExprVisitor::VisitExpr_(op);
-    AssignReturnSid(GetRef<Expr>(op));
-  }
+  void VisitExpr_(const VarNode* op) final { AssignReturnSid(GetRef<Expr>(op)); }
 
   void VisitExpr_(const FunctionNode* op) final {
     // do not recurse into sub function.
@@ -218,7 +215,7 @@ class AOTOnDemandAllocator : public ExprVisitor {
 };
 
 /*! \brief Code generator for AOT executor */
-class AOTExecutorCodegen : public ExprVisitor {
+class AOTExecutorCodegen : public MixedModeVisitor {
  protected:
   /*!
    * \brief Utility function to allocate a DLTensor or TVMValue
@@ -437,7 +434,6 @@ class AOTExecutorCodegen : public ExprVisitor {
   void VisitExpr_(const OpNode* op) override {
     throw std::runtime_error("can not compile op in non-eta expanded form");
   }
-  void VisitExpr_(const GlobalVarNode* op) override { throw std::runtime_error(""); }
   void VisitExpr_(const IfNode* op) override { throw std::invalid_argument("if not supported"); }
   void VisitExpr_(const FunctionNode* op) override {
     ICHECK(op->GetAttr<String>(attr::kCompiler).defined())