Function dice_score 

pub fn dice_score(
    predictions: &[usize],
    targets: &[usize],
    num_classes: usize,
) -> Vec<f32>

Per-class Dice coefficient: 2 * |pred ∩ target| / (|pred| + |target|).

Returns a Vec of Dice scores, one per class. Classes that appear in neither predictions nor targets receive a score of 0.0.