public WordPieceEmbedder build() {
            if (models.isEmpty()) throw new IllegalStateException("At least one model must be supplied");
            return new WordPieceEmbedder(this);
        }
Adds the model that will be used if the language is unknown, OR only one model is specified.
The same as addModel(Language.UNKNOWN, model).
linguistics-components/src/main/java/com/yahoo/language/wordpiece/WordPieceEmbedder.java