Function unsplit_head 

pub fn unsplit_head(
    d: &[f32],
    t: usize,
    c: usize,
    h: usize,
    which: usize,
) -> Vec<f32>

split_heads backward for one of q/k/v: place d [h, t, hd] into the which third of a zeroed [t, 3c] buffer.