Change size_t to int

To fix the compiler error on Mac OSX:

/dgl/src/c_api_common.cc:27:16: error: conversion from 'tvm::runtime::TVMArgValue' to 'size_t' (aka 'unsigned long') is ambiguous
        size_t which = args[0];

src/c_api_common.cc

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