private boolean hasSampleInformation() {
        if (trees.size() == 0) return true; // no matter
        return trees.get(0).samples() !=null;
    }
Return whether this model has sample information.
Don't bother to check every node as files either has this for all nodes or for none.
searchlib/src/main/java/com/yahoo/searchlib/gbdt/GbdtModel.java