Function run_while 

pub fn run_while(
    cache: &mut SubgraphCache,
    backend: &dyn Backend,
    cond: &Graph,
    body: &Graph,
    initial: Vec<Vec<f32>>,
    input_names: &[&str],
    max_iterations: Option<usize>,
) -> Vec<Vec<f32>>

Helper: evaluate an Op::While by repeatedly running cond + body. loop_carried are the values flowing through iterations.