Add const for OpDesc::id() and VarDesc::id() (#36298)

* add const OpDesc id()

* add const for VarDesc::id()

paddle/fluid/framework/op_desc.h

@@ -164,7 +164,7 @@ class OpDesc {
 
   // Note: the identity only used as a key for referring to its
   // distributed attribute now.
-  uint64_t Id() { return id_; }
+  uint64_t Id() const { return id_; }
 
  private:
   template <typename MapType>

paddle/fluid/framework/var_desc.h

@@ -160,7 +160,7 @@ class VarDesc {
 
   // Note: the identity only used as a key for referring to its
   // distributed attribute now.
-  uint64_t Id() { return id_; }
+  uint64_t Id() const { return id_; }
 
  private:
   const proto::VarType::TensorDesc &tensor_desc() const;