Function dense_gemm_splitk 

pub fn dense_gemm_splitk(
    gpu: &dyn GpuBackend,
    partial_kernel: KernelHandle,
    reduce_kernel: KernelHandle,
    input: DevicePtr,
    weight: &DenseWeight,
    output: DevicePtr,
    workspace: DevicePtr,
    m: u32,
    n: u32,
    k: u32,
    k_splits: u32,
    stream: u64,
) -> Result<()>

2026-09-25: Split-K BF16 GEMM: partial_kernel writes FP32 partial products for k_splits K chunks into workspace, and reduce_kernel sums them into BF16 output. workspace holds k_splits * m * n FP32 values.