Function mixup 

pub fn mixup(
    x1: &ArrayD<f64>,
    x2: &ArrayD<f64>,
    alpha: f64,
    rng: &mut AugRng,
) -> Result<(ArrayD<f64>, f64), AugmentationError>

Mixup: λ·x1 + (1−λ)·x2 where λ ~ Beta(alpha, alpha).

Returns (mixed, lambda).