Adding model domain to ONNX model export.

Source/CNTKv2LibraryDll/proto/onnx/CNTKToONNX.cpp

@@ -410,7 +410,8 @@ std::unique_ptr<ONNXIR::Model> CNTKToONNX::CreateModel(const FunctionPtr& src)
     if (!status.IsOK())
         LogicError("%s", status.ErrorMessage().c_str());
 
-    model->SetModelversion(static_cast<ONNXIR::Version>(CNTK_ONNX_MODEL_VERSION)); // This is the default. Should be surfaced as graph's 'save' API input.
+    model->SetModelversion(static_cast<ONNXIR::Version>(CNTK_ONNX_MODEL_VERSION)); // REVIEW sptiwari: This is the default. This and doc_string should be surfaced as graph's 'save' API input.
+    model->SetDomain(CNTK_ONNX_MODEL_DOMAIN);
     model->SetProducerVersion(CNTK_ONNX_PRODUCER_VERSION);
     model->SetProducerName(CNTK_ONNX_PRODUCER_NAME);
     return model;

Source/CNTKv2LibraryDll/proto/onnx/CNTKToONNX.h

@@ -11,6 +11,7 @@
 #define CNTK_ONNX_MODEL_VERSION 1
 #define MACRO_TO_STRING(s) #s
 const std::string CNTK_ONNX_PRODUCER_NAME = "CNTK";
+const std::string CNTK_ONNX_MODEL_DOMAIN = "ai.cntk";
 #ifdef _WIN32
 const std::string CNTK_ONNX_PRODUCER_VERSION = CNTK_VERSION;
 #else