Function gemm_many_keep 

pub fn gemm_many_keep(
    model: &Arc<CmfModel>,
    idxs: &[(usize, usize)],
    xs: &[f32],
    b: usize,
    cols: usize,
    out: &mut [f32],
) -> bool

Several projections of one host activation, read back in one trip: out = [P0 | P1 | …], each idxs entry (tensor, rows). wgpu only.