vulkan: Implement split_k for coopmat2 flash attention. (#12627)

When using group query attention, we have one workgroup per KV batch and this
can be very few workgroups (e.g. just 8 in some models). Enable split_k to
spread the work across SMs. This helps a lot when the KV cache is large.

ggml/src/ggml-vulkan/vulkan-shaders/flash_attn_split_k_reduce.comp

@@ -0,0 +1,59 @@
+#version 450
+
+#extension GL_EXT_control_flow_attributes : enable
+
+#define BLOCK_SIZE 32
+
+layout(local_size_x = BLOCK_SIZE, local_size_y = 1, local_size_z = 1) in;
+
+layout (binding = 0) readonly buffer A {float data_a[];};
+layout (binding = 1) writeonly buffer D {float data_d[];};
+
+layout (push_constant) uniform parameter {
+    uint D;
+    uint N;
+    uint k_num;
+} p;
+
+void main() {
+    // Each workgroup handles a row
+    const uint n = gl_WorkGroupID.x;
+    const uint tid = gl_LocalInvocationID.x;
+
+    uint D = p.D;
+    uint N = p.N;
+    uint k_num = p.k_num;
+
+    uint l_offset = D * N * k_num + n;
+    uint m_offset = D * N * k_num + N + n;
+    uint lm_stride = N * 2;
+
+    // Compute the max m value for the row
+    float m_max = -1.0/0.0;
+    [[unroll]] for (uint k = 0; k < k_num; ++k) {
+        float m = data_a[m_offset + k * lm_stride];
+        m_max = max(m_max, m);
+    }
+
+    // Compute L based on m_max
+    float L = 0;
+    [[unroll]] for (uint k = 0; k < k_num; ++k) {
+        float l = data_a[l_offset + k * lm_stride];
+        float m = data_a[m_offset + k * lm_stride];
+        L += exp(m - m_max) * l;
+    }
+
+    L = 1.0 / L;
+
+    // Scale and sum the O contributions based on m_max and store the result to memory
+    for (uint d = tid; d < D; d += BLOCK_SIZE) {
+        float O = 0.0;
+        [[unroll]] for (uint k = 0; k < k_num; ++k) {
+            uint o_offset = D * N * k + D * n + d;
+            float m = data_a[m_offset + k * lm_stride];
+            O += exp(m - m_max) * data_a[o_offset];
+        }
+        O *= L;
+        data_d[D * n + d] = O;
+    }
+}