Function bottleneck_3x3 

pub fn bottleneck_3x3<D: Float + 'static>(
    c: usize,
    shortcut: bool,
) -> impl Fn(SymTensor) -> SymTensor

conv(c → c, k=3) → conv(c → c, k=3).

Used as the inner bottleneck inside C3k blocks (e=1.0).