public static GbdtModel fromXml(String xml) throws ParserConfigurationException, IOException, SAXException {
        return fromDom(XmlHelper.parseXml(xml));
    }
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