fix: correct prefill mask index, unify GQA kernel interface
- Fix mask indexing: batch*q_len*kv_len -> batch*kv_len
- Add csrc/kernels/gqa_common.cuh with shared GQAParams struct
- Unify decode/prefill Python API: both accept (q,k,v,mask=None,...)
- Decode now supports optional mask, is_causal, causal_offset, scale
- Rename struct fields: B->batch, Hq->q_head, Hk->kv_head, D->head_dim
- Use py::arg() for correct None/defaults handling in pybind11
- Update pure C tests and build instructions (-arch=sm_89)
ViperEkura
11fa807cfc