[VitisAI] add new api to VitisAI to save graph as a string (microsoft#25602)

### Description
Add new API to VitisAI to save graph as a string

### Motivation and Context
to support in-memory flow

---------

Co-authored-by: yifei <[email protected]>

Author: yifei410

onnxruntime/core/providers/vitisai/imp/global_api.cc

@@ -580,6 +580,8 @@ vaip_core::OrtApiForVaip* create_org_api_hook() {
     graph.RemoveInitializedTensor(tensor_name);
   };
   the_global_api.graph_reverse_dfs_from_preemp = vaip::graph_reverse_dfs_from;
+  the_global_api.graph_save_string = vaip::graph_save_string;
+
   if (!s_library_vitisaiep.vaip_get_version) {
     return reinterpret_cast<vaip_core::OrtApiForVaip*>(&(the_global_api.host_));
   } else {

onnxruntime/core/providers/vitisai/imp/graph.cc

@@ -205,6 +205,29 @@ void graph_save(const Graph& graph, const std::string& filename, const std::stri
   vai_assert(result, "model serialize to ostream error");
 }
 
+vaip_core::DllSafe<std::string> graph_save_string(const Graph& graph) {
+  auto model_proto = const_cast<onnxruntime::Model&>(graph.GetModel()).ToProto();
+  auto graph_proto_subgraph = graph.ToGraphProto();
+  *model_proto->mutable_graph() = *graph_proto_subgraph;
+  auto& logger = logging::LoggingManager::DefaultLogger();
+  auto model = Model::Create(std::move(*model_proto), graph.ModelPath(), nullptr, logger);
+  model_proto = model->ToProto();
+  auto& metadata = model->MetaData();
+  if (!metadata.empty()) {
+    auto metadata_props = model_proto->mutable_metadata_props();
+    metadata_props->Clear();
+    for (auto& m : metadata) {
+      auto prop = metadata_props->Add();
+      *prop->mutable_key() = m.first;
+      *prop->mutable_value() = m.second;
+    }
+  }
+  std::string graph_string;
+  bool result = model_proto->SerializeToString(graph_string);
+  vai_assert(result, "model serialize to string error");
+  return vaip_core::DllSafe(graph_string);
+}
+
 Node& graph_fuse(Graph& graph, const std::string& name,
                  const std::string& op_type,
                  const std::vector<size_t>& nodes,

onnxruntime/core/providers/vitisai/include/vaip/graph.h

@@ -14,6 +14,7 @@ Node& graph_add_node(Graph& graph, const std::string& name, const std::string& o
                      const NodeAttributes& attributes, const std::string& domain);
 void graph_save(const Graph& graph, const std::string& filename, const std::string& dat_filename,
                 size_t initializer_size_threshold);
+vaip_core::DllSafe<std::string> graph_save_string(const Graph& graph);
 Node& graph_fuse(Graph& graph, const std::string& name, const std::string& op_type, const std::vector<size_t>& nodes,
                  const std::vector<std::string>& inputs, const std::vector<std::string>& outputs,
                  const std::vector<std::string>& constant_initializers);

onnxruntime/core/providers/vitisai/include/vaip/vaip_ort_api.h

@@ -13,7 +13,7 @@ struct OrtApi;
 
 namespace vaip_core {
 
-#define VAIP_ORT_API_MAJOR (17u)
+#define VAIP_ORT_API_MAJOR (18u)
 #define VAIP_ORT_API_MINOR (0u)
 #define VAIP_ORT_API_PATCH (0u)
 struct OrtApiForVaip {
@@ -252,10 +252,11 @@ struct OrtApiForVaip {
           stop);                                           // [103]
   void (*graph_set_name)(Graph& graph, const char* name);  // [104]
   void (*graph_infer_shapes_from_filepath)(
-      const std::string& m, const std::string& save_path);  // [105]
-  GraphProto* (*graph_to_graph_proto)(const Graph& graph);  // [106]
-  void (*graph_proto_delete)(GraphProto* p);                // [107]
-  void (*graph_infer_shapes)(ModelProto& m);                // [108]
+      const std::string& m, const std::string& save_path);        // [105]
+  GraphProto* (*graph_to_graph_proto)(const Graph& graph);        // [106]
+  void (*graph_proto_delete)(GraphProto* p);                      // [107]
+  void (*graph_infer_shapes)(ModelProto& m);                      // [108]
+  DllSafe<std::string> (*graph_save_string)(const Graph& graph);  // [109]
 };
 
 #ifndef USE_VITISAI