private ExpressionNode transformCustomTokenInputIds(ReferenceNode feature, RankProfileTransformContext context) {
        ExpressionNode cls = feature.getArguments().expressions().get(0);
        ExpressionNode sep = feature.getArguments().expressions().get(1);
        return transformTokenInputIds(feature, context, cls, sep, 3);
    }
Transforms a feature of the form

    customTokenInputIds(1, 2, 128, a, b, ...)

to an expression that concatenates the arguments a, b, ... using the
first and second arguments as the CLS and SEP padding tokens, here
1 and 2, respectively. Otherwise, identical to tokenInputIds.
config-model/src/main/java/com/yahoo/schema/expressiontransforms/TokenTransformer.java