private ExpressionFunction.Instance lengthFunctionFor(ReferenceNode arg) {
        var ctx = new SerializationContext();
        return commonLengthFunction.expand(ctx, List.of(arg), new ArrayDeque<String>());
    }
Transforms a feature of the form

    tokenAttentionMask(128, a, b, ...)

to an expression that generates a tensor that has values 1 for all
arguments (including CLS and SEP tokens) and 0 for the rest of the
sequence.

Concretely, transforms to a tensor generation expression:

    tensor(d0[1],d1[128])(if(d1 < 1 + length_a + 1 + length_b + 1 + ..., 1, 0))
config-model/src/main/java/com/yahoo/schema/expressiontransforms/TokenTransformer.java