PER_TOKEN_HEAD batch prefill reads q_descale_per_token for a full kM0
query tile. For small sequence lengths or chunked-prefill tail chunks,
some tile rows are padding rows beyond the valid query range, but the
kernel still indexed q_descale_per_token_ptr for those rows. In group
mode, q_descale_ptr also missed the per-batch query_start offset, so
later batches could read descales from the wrong position.
Fix the issue by offsetting q_descale_ptr with query_start, exposing
the mask's valid query row count, and guarding q_descale_per_token
loads for rows outside y_total.
Patch credits: Zhen Han <zhen.han@amd.com>
2867294db3