Function topk_to_mask 

pub fn topk_to_mask(
    topk: &[i32],
    q_rows: usize,
    width: usize,
    kv_len: usize,
) -> Vec<u8> 

Turn an index row into the boolean visibility mask the attention consumes.

Mirrors HF’s scatter_add + .ne(0): duplicate indices collapse, so a repeated token is attended once, not twice. Out-of-range and INVALID entries are dropped.