Function unknown_cost 

pub fn unknown_cost(
    _shapes: &[Shape],
    _dtypes: &[DType],
    _params: &OpParams,
    _caps: &BackendCapabilities,
) -> CostEstimate

Sentinel “no cost claim” function — returns crate::fused::CostEstimate::default (all-zero). The fill pass (fill_unset_cpu_cost) recognizes this exact function pointer to decide which entries get the OpKind-family default.