[Bugfix] 'const size_t' (aka 'const unsigned long') is ambiguous on MacOS (#363)

[Bugfix] 'const size_t' (aka 'const unsigned long') is ambiguous on MacOS

src/c_api_common.cc

@@ -24,7 +24,7 @@ DLManagedTensor* CreateTmpDLManagedTensor(const DGLArgValue& arg) {
 
 PackedFunc ConvertNDArrayVectorToPackedFunc(const std::vector<NDArray>& vec) {
     auto body = [vec](DGLArgs args, DGLRetValue* rv) {
-        const size_t which = args[0];
+        const int which = args[0];
         if (which >= vec.size()) {
             LOG(FATAL) << "invalid choice";
         } else {

src/graph/graph_apis.cc

@@ -70,7 +70,7 @@ PackedFunc ConvertSubgraphToPackedFunc(const Subgraph& sg) {
 // Convert Sampled Subgraph structures to PackedFunc.
 PackedFunc ConvertSubgraphToPackedFunc(const std::vector<SampledSubgraph>& sg) {
   auto body = [sg] (DGLArgs args, DGLRetValue* rv) {
-      const size_t which = args[0];
+      const int which = args[0];
       if (which < sg.size()) {
         GraphInterface* gptr = sg[which].graph->Reset();
         GraphHandle ghandle = gptr;