refactor: template AttentionParams, rename .cuh to .h

- Convert AttentionParams to a template struct supporting arbitrary types
- Rename attn_common.cuh -> attn_common.h (no CUDA-specific code remains)
- Include standard headers explicitly in each .cuh instead of via attn_common.cuh
- Allow .h files in csrc/ via .gitignore

csrc/kernels/attn_prefill_split_q_mma.cuh

@@ -1,7 +1,11 @@
 #pragma once
-#include "attn_common.cuh"
+#include <cfloat>
+#include <cuda_bf16.h>
+#include "attn_common.h"
 #include "attn_mma_utils.cuh"
 
+using bf16 = __nv_bfloat16;
+
 // Tensor-core prefill flash attention (raw mma.sync PTX).
 // One warp owns BR=16 query rows. S = Q@K^T and O = P@V run on bf16 tensor
 // cores via mma.sync.m16n8k16 (f32 accumulate). Q fragments are loaded once
@@ -38,7 +42,7 @@
 
 template <int HEAD_DIM, int WARPS, int BC, int MIN_BLOCKS>
 __global__ __launch_bounds__(WARPS * 32, MIN_BLOCKS)
-void attn_prefill_split_q_mma_kernel(AttentionParams p) {
+void attn_prefill_split_q_mma_kernel(AttentionParams<bf16> p) {
     constexpr int BR = 16;
     constexpr int KD = HEAD_DIM / 16;  // Q/K k-tiles
     constexpr int NC8 = BC / 8;        // S n-tiles (N=8 each)