refactor: stride-based attn interface with layout and causal mask

- Replace is_causal + causal_offset with unified causal_offset (-1 = off, >=0 = first Q pos)
- Causal and mask can now coexist (was mutually exclusive)
- Add stride-based addressing for Q/KV/O (layout-agnostic, zero-copy)
- Add layout param ("bhld"/"blhd") parsed in Python, passed as int to C++
- Support 2D [batch, kv_len] and 3D [batch, q_len, kv_len] mask
- Vectorize paged KV gather in Python fallback (was per-token Python loop)
- Extract shared helpers: compute_num_splits, alloc_split_partials, dispatch_head_dim
- Unify paged_decode entry via attn_pack_paged_params
- Update mma_softmax_tile for 3D mask with per-row qrow indexing

csrc/kernels/attn_common.h

@@ -10,11 +10,19 @@ struct AttentionParams {
     int kv_len;
     int head_dim;
     int use_mask;
-    int is_causal;
-    int causal_offset;
+    int causal_offset;   // -1 = non-causal; >=0 = absolute position of first Q token
     int num_splits;
     float scale;
 
+    // Q strides (element offsets for each dim — layout-agnostic)
+    int q_stride_b, q_stride_h, q_stride_l, q_stride_d;
+    // KV strides (K and V share the same layout — only base pointers differ)
+    int kv_stride_b, kv_stride_h, kv_stride_l, kv_stride_d;
+
+    // Mask: 2D [batch, kv_len] (mask_q_stride=0) or 3D [batch, q_len, kv_len]
+    int mask_b_stride;   // = kv_len (both 2D and 3D)
+    int mask_q_stride;   // 2D: 0 (all q rows share); 3D: kv_len
+
     const T* __restrict__ q;
     const T* __restrict__ k;
     const T* __restrict__ v;
@@ -34,7 +42,6 @@ struct PagedAttentionParams {
     int kv_len;
     int head_dim;
     int use_mask;
-    int is_causal;
     int causal_offset;
     float scale;
 
@@ -42,12 +49,19 @@ struct PagedAttentionParams {
     int page_size;
     int max_pages;
 
+    // Q strides (layout-agnostic)
+    int q_stride_b, q_stride_h, q_stride_l, q_stride_d;
+
+    // Mask strides (2D or 3D)
+    int mask_b_stride;
+    int mask_q_stride;
+
     const T* __restrict__ q;
     const T* __restrict__ k_cache;
     const T* __restrict__ v_cache;
     const bool* __restrict__ mask;
     const int64_t* __restrict__ page_table;
-    
+
     T* __restrict__ o;
     AT* __restrict__ o_part;
     AT* __restrict__ ml_part;