Multi produce functions possibly cause failure to Function_call.get_ys()

[carrotflakes]

Reproduce:

    let x = backprop(NDArray::zeros(&[2, 3][..]));
    let ys = x.slices(vec![ndarray::s![0, ..], ndarray::s![1, ..]]);
    let y0 = ys[0].clone();
    drop(ys);
    optimize(&y0);