Prevent unused variables from generating warnings across all platform…

…s. (onnx#1930)

* Change the return type for the zipmap operator to match the description in the spec.

* Prevent unused variables from generating warnings across all platforms.
This was observed in onnxruntime when __ONNX_NO_DOC_STRINGS was enabled.

* nit change

onnx/common/common.h

@@ -0,0 +1,12 @@
+// Copyright (c) ONNX Project Contributors.
+// Licensed under the MIT license.
+
+#pragma once
+
+namespace ONNX_NAMESPACE {
+#ifdef _WIN32
+#define ONNX_UNUSED_PARAMETER(x) (x)
+#else
+#define ONNX_UNUSED_PARAMETER(x) (void)(x)
+#endif
+} // namespace ONNX_NAMESPACE

onnx/defs/schema.h

@@ -20,6 +20,7 @@
 
 #include "data_type_utils.h"
 #include "onnx/common/constants.h"
+#include "onnx/common/common.h"
 #include "onnx/defs/shape_inference.h"
 #include "onnx/onnx-operators_pb.h"
 namespace ONNX_NAMESPACE {
@@ -265,7 +266,7 @@ class OpSchema final {
 #ifndef __ONNX_NO_DOC_STRINGS
     SetDoc(std::string(doc));
 #else
-    doc;
+    ONNX_UNUSED_PARAMETER(doc);
 #endif
 
     return *this;