ggml-webgpu: fix compiler warnings and refactor FlashAttention encoding (#21052)

* Update workflows to remove dependence on llvmpipe

* Try setting Dawn_DIR

* remove c++20 initializers

* Move to proper guid

* Try avoiding segfaults on vulkan backend process exit

* Remove compiler warnings on parameter casting

* Fix soft_max and update reg_tile accumulation to f32 for better precision

* Refactor flash_attn a bit

* remove c++20 initializers and format

* Increase div precision for NVIDIA

* revert div precision and comment out ggml-ci node for now

* Formatting

* Try debugging on a failing CI node

* Revert "Try debugging on a failing CI node"

This reverts commit 1971e33cba919915e12bcfd5828abfbd54ca942e.

ggml/src/ggml-webgpu/wgsl-shaders/flash_attn_vec_blk.wgsl

@@ -1,7 +1,6 @@
 diagnostic(off, subgroup_uniformity);
 enable f16;
 
-#define Q_TILE 1
 #define KV_TILE 32
 #define WG_SIZE 32
 
@@ -11,7 +10,7 @@ struct Params {
     seq_len_kv: u32,
     stride_mask3: u32,
     // Number of KV blocks and Q blocks per batch.
-    // nblk0 = ceil(seq_len_kv / KV_TILE), nblk1 = ceil(seq_len_q / Q_TILE).
+    // nblk0 = ceil(seq_len_kv / KV_TILE), nblk1 = seq_len_q.
     nblk0: u32,
     nblk1: u32,
 };
@@ -40,7 +39,7 @@ fn main(@builtin(workgroup_id) wg_id: vec3<u32>,
         return;
     }
 
-    let q_start = q_blk * Q_TILE;
+    let q_start = q_blk;
     let k_start = kv_blk * KV_TILE;
 
     let mask_batch = select(0u, batch_idx, params.stride_mask3 > 0u);
@@ -54,11 +53,8 @@ fn main(@builtin(workgroup_id) wg_id: vec3<u32>,
     var local_max = -MASK_MAX;
     var local_any = 0u;
 
-    for (var q_rel = 0u; q_rel < Q_TILE; q_rel += 1u) {
-        let q_row = q_start + q_rel;
-        if (q_row >= params.seq_len_q) {
-            continue;
-        }
+    let q_row = q_start;
+    if (q_row < params.seq_len_q) {
         let row_base = mask_batch_base + q_row * params.seq_len_kv;
         for (var k_rel = local_id.x; k_rel < KV_TILE; k_rel += WG_SIZE) {
             let k_col = k_start + k_rel;