Adds GQA forward and boolean mask/bias

Adds forward support for GQA/MQA (different Q vs KV heads) with optional boolean mask and bias, including broadcasting across batch/head/seq dims and per-head routing.

Switches to compile-time mask/bias flags, removes the scratchpad workaround, simplifies scaling, and indexes LSE/Out by Q heads. Skips masked-out tiles, tightens mask semantics (True = keep), and fixes backward mask handling.

Introduces a contiguity helper, bumps pipeline stages, and errors out on Triton backward for GQA/MQA until implemented.

Author: LoserCheems