tree.h: Fix SetModelPresetVariant variant_index template parameter (#600)

* tree.h: Fix SetModelPresetVariant variant_index template parameter

Fixes compiling with -Werror=sign-conversion
Fixes #599

* Update include/treelite/tree.h

---------

Co-authored-by: Philip Hyunsu Cho <chohyu01@cs.washington.edu>

Author: davidgraeff

include/treelite/tree.h

@@ -428,8 +428,8 @@ class ModelPreset {
 
 using ModelPresetVariant = std::variant<ModelPreset<float, float>, ModelPreset<double, double>>;
 
-template <int variant_index>
-ModelPresetVariant SetModelPresetVariant(int target_variant_index) {
+template <std::size_t variant_index>
+ModelPresetVariant SetModelPresetVariant(std::size_t target_variant_index) {
   ModelPresetVariant result;
   if constexpr (variant_index != std::variant_size_v<ModelPresetVariant>) {
     if (variant_index == target_variant_index) {