Function conv_plain 

pub fn conv_plain<D: Float>(
    c_in: usize,
    c_out: usize,
    k: usize,
    s: usize,
) -> impl Fn(SymTensor) -> SymTensor

Plain Conv2d with bias — no BatchNorm, no activation.

Matches nn.Conv2d(c_in, c_out, k, bias=True) from the ultralytics Detect head.