Fix compiler warning about signed vs unsigned ints

[hcho3]

src/cpp/src/fil/treelite_import.cu:496:26: warning: comparison of integer expressions of different signedness: 'size_t' {aka 'long unsigned int'} and 'const int' [-Wsign-compare]
103.2   496 |     ASSERT(leaf_vec_size == model.num_class[0], "treelite model inconsistent");
103.2       |           ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~
103.2 /rapids_triton/build/_deps/cuml-src/cpp/src/fil/treelite_import.cu:516:40: warning: comparison of integer expressions of different signedness: 'const int' and 'size_t' {aka 'long unsigned int'} [-Wsign-compare]
103.2   516 |         ASSERT(model.class_id[tree_id] == tree_id % static_cast<size_t>(model.num_class[0]),
103.2       |           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[codecov-commenter]

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```diff @@ Coverage Diff @@ ## branch-24.10 #6053 +/- ## =============================================== Coverage ? 67.06% =============================================== Files ? 195 Lines ? 12920 Branches ? 0 =============================================== Hits ? 8665 Misses ? 4255 Partials ? 0 ```

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[divyegala]

/merge