metal : add sparse FA (#28098)

* metal : support n_kv_max sparse mask hint in flash attention vec kernel

- add kernel_flash_attn_ext_vec_idx: compacts finite mask entries into
  a per-row index list (Hillis-Steele scan, one threadgroup per row)
- extend vec FA kernel with optional sparse index gathering (FC slot 5)
- add host-side gate: sparse path when n_kv_max > 0, mask present,
  supported head sizes / KV types, n_kv_max <= 4096
- new buffer region extra_idx for the index list
- pipeline getter extended with has_sparse param
- add test cases: head sizes, quant types, nb>1, nr23 variants,
  sinks, ALiBi, softcap, permute, v_view_of_k, no-mask fallback

Note: multi-row (nb*nr23[1] > 1) cases still failing - rid mapping
in the store phase needs revisiting for the sparse path.

Assisted-by: pi:llama.cpp/Qwen3.8-27B

* metal : fix sparse flash attention row addressing

- kernel_flash_attn_ext_vec_idx: mask param is half* but nb31 is a byte
  stride, so the per-row mask offset was scaled by 2x; cast to char*
  before applying the byte strides
- kernel_flash_attn_ext_vec: sparse pidx param is char* so the per-row
  element offset was under-scaled by sizeof(int); scale it by sizeof(int)
  to get the correct byte offset
- fixes the multi-row (nb*nr23[1] > 1) sparse flash attention failures

Assisted-by: pi:llama.cpp/DeepSeek-v4-0731

* cont : use sparse vec FA for prefill

* metal : single-pass flash attention sparse index compaction

The idx kernel previously read the mask row twice: once to count the finite
entries (for the prefix scan) and again to recover their positions. Since the
kernel is memory-bound, this doubled the mask traffic.

Keep the finite positions in a per-thread register array during the count
pass and write them out directly, avoiding the second mask read. A dense
mask with more than NLOCAL finite entries in a slice falls back to re-reading
the mask to write the remaining positions.

Assisted-by: pi:llama.cpp/DeepSeek-v4-0731

* tests : add perf cases for sparse flash attention prefill

Measure the sparse vec FA kernel across KV sizes, n_kv_max hints and batch
sizes. Run with:

    ./build/bin/test-backend-ops -b MTL0 -o FLASH_ATTN_EXT -p "n_kv_max=[1-9]" perf

Assisted-by: pi:llama.cpp/DeepSeek-v4-0731

* qwen4 : enable sparse attention

* cont : adjust nsg

* cont : sync test-backend-ops

* cont : disable Qwen4 for now

* cont : clean-up + tests

ggml/src/ggml-metal/ggml-metal-device.cpp

@@ -1577,6 +1577,26 @@ ggml_metal_pipeline_with_params ggml_metal_library_get_pipeline_flash_attn_ext(
     return res;
 }
 
+ggml_metal_pipeline_with_params ggml_metal_library_get_pipeline_flash_attn_ext_vec_idx(
+        ggml_metal_library_t lib,
+        const ggml_tensor * op) {
+    assert(op->op == GGML_OP_FLASH_ATTN_EXT);
+    assert(op->src[3]);
+
+    char name[256];
+
+    snprintf(name, 256, "kernel_flash_attn_ext_vec_idx");
+
+    ggml_metal_pipeline_with_params res = ggml_metal_library_get_pipeline(lib, name);
+    if (!res.pipeline) {
+        res = ggml_metal_library_compile_pipeline(lib, name, name, nullptr);
+    }
+
+    GGML_UNUSED(op);
+
+    return res;
+}
+
 ggml_metal_pipeline_with_params ggml_metal_library_get_pipeline_flash_attn_ext_vec(
         ggml_metal_library_t lib,
         const ggml_tensor * op,
@@ -1585,6 +1605,7 @@ ggml_metal_pipeline_with_params ggml_metal_library_get_pipeline_flash_attn_ext_v
         bool    has_bias,
         bool    has_scap,
         bool    has_kvpad,
+        bool    has_sparse,
         int32_t nqpsg,
         int32_t ne,
         int32_t nsg,
@@ -1614,13 +1635,14 @@ ggml_metal_pipeline_with_params ggml_metal_library_get_pipeline_flash_attn_ext_v
             dv,
             qne_suffix);
 
-    snprintf(name, 256, "%s_mask=%d_sink=%d_bias=%d_scap=%d_kvpad=%d_ns10=%d_ns20=%d_nsg=%d_nwg=%d",
+    snprintf(name, 256, "%s_mask=%d_sink=%d_bias=%d_scap=%d_kvpad=%d_sparse=%d_ns10=%d_ns20=%d_nsg=%d_nwg=%d",
             base,
             has_mask,
             has_sinks,
             has_bias,
             has_scap,
             has_kvpad,
+            has_sparse,
             ns10,
             ns20,
             nsg, nwg);
@@ -1633,7 +1655,8 @@ ggml_metal_pipeline_with_params ggml_metal_library_get_pipeline_flash_attn_ext_v
         ggml_metal_cv_set_bool(cv, has_sinks, FC_FLASH_ATTN_EXT_VEC + 1);
         ggml_metal_cv_set_bool(cv, has_bias,  FC_FLASH_ATTN_EXT_VEC + 2);
         ggml_metal_cv_set_bool(cv, has_scap,  FC_FLASH_ATTN_EXT_VEC + 3);
-        ggml_metal_cv_set_bool(cv, has_kvpad, FC_FLASH_ATTN_EXT_VEC + 4);
+        ggml_metal_cv_set_bool(cv, has_kvpad,  FC_FLASH_ATTN_EXT_VEC + 4);
+        ggml_metal_cv_set_bool(cv, has_sparse, FC_FLASH_ATTN_EXT_VEC + 5);
 
         ggml_metal_cv_set_int32(cv, ns10, FC_FLASH_ATTN_EXT_VEC + 20);
         ggml_metal_cv_set_int32(cv, ns20, FC_FLASH_ATTN_EXT_VEC + 21);