refactor: rename gqa_* to attn_*, split-KV for all decode paths

- Rename all csrc/kernels/gqa_*.cuh/cu to attn_*, with _split_q / _split_kv
  strategy suffix and optional _mma compute suffix
- Remove non-split MMA decode kernel, keep only split-KV path
- Convert scalar decode fallback to split-KV (o_part/ml_part + combine)
- Move combine kernel to attn_decode_split_kv.cuh (shared by both paths)
- Rename GQAParams to AttentionParams
- Update all C++ #include, PYBIND11, and Python extension references

csrc/build.py

@@ -42,5 +42,5 @@ def register(name: str, sources: list[str] | None = None, **kwargs):
     }
 
 
-register("gqa_decode_attn")
-register("gqa_prefill_attn")
+register("attn_decode")
+register("attn_prefill")