message: Enable type tags for GS TreeModels
  Summary: Assert that all mergeables in an external type tag model must have the ModelIdendity annotation.
diff: @@ -40,6 +40,8 @@ TypeTags collect_type_tags(const std::vector<const MergerType*>& mergers) {
  for (auto merger : mergers) {
  for (const auto type : merger->mergeables) {
  auto type_tag = type_tag_utils::parse_model_type_tag(type_class(type));
  + always_assert_log(
  + type_tag != boost::none, "Type tag is missing from %s\n", SHOW(type));
  type_tags.set_type_tag(type, *type_tag);
  }
  }
  