fix convert_to_mixed_precision api save model bug (#52767)

* update save model

* update

paddle/fluid/inference/analysis/passes/convert_to_mixed_precision.cc

@@ -102,32 +102,53 @@ void ConvertToMixedPrecisionPass::SaveMixedModel() {
   framework::ProgramDesc mixed_program_desc;
   framework::ir::GraphToProgram(*main_graph_, &mixed_program_desc);
 
-  auto parameters = scope_.LocalVarNames();
-  std::sort(parameters.begin(), parameters.end());
-
-  auto SerializeParams = [&]() -> std::string {
-    std::ostringstream os;
-    phi::CPUContext ctx;
-    for (const auto& param : parameters) {
-      PADDLE_ENFORCE_NOT_NULL(
-          scope_.FindVar(param),
-          platform::errors::NotFound(
-              "Block should already have a '%s' variable", param));
-      auto* tensor = scope_.FindVar(param)->GetMutable<phi::DenseTensor>();
-      framework::SerializeToStream(os, *tensor, ctx);
+  auto SerializeParams = [&](const std::string& path) {
+    auto IsPersistable = [](const framework::VarDesc* var) {
+      if (var->Persistable() &&
+          var->GetType() != framework::proto::VarType::FEED_MINIBATCH &&
+          var->GetType() != framework::proto::VarType::FETCH_LIST &&
+          var->GetType() != framework::proto::VarType::RAW) {
+        return true;
+      }
+      return false;
+    };
+    framework::ProgramDesc save_program;
+    auto* save_block = save_program.MutableBlock(0);
+
+    const auto& global_block = mixed_program_desc.Block(0);
+    std::vector<std::string> save_var_list;
+    for (framework::VarDesc* var : global_block.AllVars()) {
+      if (IsPersistable(var)) {
+        framework::VarDesc* new_var = save_block->Var(var->Name());
+        new_var->SetShape(var->GetShape());
+        new_var->SetDataType(var->GetDataType());
+        new_var->SetType(var->GetType());
+        new_var->SetLoDLevel(var->GetLoDLevel());
+        new_var->SetPersistable(true);
+
+        save_var_list.push_back(new_var->Name());
+      }
     }
-    return os.str();
+    std::sort(save_var_list.begin(), save_var_list.end());
+    auto* op = save_block->AppendOp();
+    op->SetType("save_combine");
+    op->SetInput("X", save_var_list);
+    op->SetAttr("file_path", path);
+    op->CheckAttrs();
+
+    framework::Executor exe(platform::CPUPlace{});
+    exe.Run(save_program, &scope_, 0, true, true);
   };
 
-  auto StrToBinary = [](const std::string& path, const std::string& str) {
+  auto SerializeProg = [&](const std::string& path) {
+    auto str = mixed_program_desc.Proto()->SerializeAsString();
     std::ofstream file(path.c_str(), std::ios::binary);
     file.write(str.c_str(), str.size());
     file.close();
   };
 
-  StrToBinary(mixed_model_file_,
-              mixed_program_desc.Proto()->SerializeAsString());
-  StrToBinary(mixed_params_file_, SerializeParams());
+  SerializeProg(mixed_model_file_);
+  SerializeParams(mixed_params_file_);
 }
 
 bool OpSupportPrecision(const std::string& op_type,