Coerce LightGBM models to float32

Author: hcho3

src/cpu_forest_model.h

@@ -54,8 +54,10 @@ struct ForestModel<rapids::HostMemory> {
             try {
               result = filex::import_from_treelite_model(
                   *tl_model_->base_tl_model(), filex::preferred_tree_layout,
-                  filex::index_type{}, std::nullopt,
-                  raft_proto::device_type::cpu);
+                  filex::index_type{}, false, raft_proto::device_type::cpu);
+              // TODO(hcho3): Throw a warning when coercing a double-precision
+              // floats to single-precision
+              // TODO(hcho3): Support inferencing with double-precision floats
               rapids::log_info(__FILE__, __LINE__)
                   << "Loaded model to new FIL format";
             }

src/gpu_forest_model.h

@@ -73,8 +73,10 @@ struct ForestModel<rapids::DeviceMemory> {
             try {
               result = filex::import_from_treelite_model(
                   *tl_model_->base_tl_model(), filex::preferred_tree_layout,
-                  filex::index_type{}, std::nullopt,
-                  raft_proto::device_type::gpu);
+                  filex::index_type{}, false, raft_proto::device_type::gpu);
+              // TODO(hcho3): Throw a warning when coercing a double-precision
+              // floats to single-precision
+              // TODO(hcho3): Support inferencing with double-precision floats
               rapids::log_info(__FILE__, __LINE__)
                   << "Loaded model to new FIL format";
             }