Function from_l2norm 

pub fn from_l2norm(
    spec: &L2NormSpec,
    names: &StageNames,
    target: &SpatialTarget,
) -> Result<SpatialProgram, SpatialReject>

y[r, i] = x[r, i] / sqrt(Σ_i x[r, i]² + 1e-6) over rows heads of cols — per-head L2 normalization on a FLAT 1 × rows·cols tensor.