vulkan: add v_dot2_f32_f16 support in matrix-matrix multiplication and Flash Attention (#24123)

* vulkan: add support for valve fp16 dot2 extension

* use macro for dot2 path choice

* properly check for the feature

* add dot_product abstraction to reduce preprocessor branching

ggml/src/ggml-vulkan/vulkan-shaders/dot_product_funcs.glsl

@@ -0,0 +1,27 @@
+#ifdef DOT2_F16
+#extension GL_EXT_spirv_intrinsics : require
+
+spirv_instruction(extensions = ["SPV_VALVE_mixed_float_dot_product"],
+                  capabilities = [6912], id = 6916)
+float v_dot2_f32_f16(f16vec2 a, f16vec2 b, float acc);
+
+ACC_TYPE dot_product(f16vec4 a, f16vec4 b, ACC_TYPE acc) {
+    return ACC_TYPE(v_dot2_f32_f16(a.zw, b.zw, v_dot2_f32_f16(a.xy, b.xy, float(acc))));
+}
+
+ACC_TYPE dot_product(f16vec2 a, f16vec2 b, ACC_TYPE acc) {
+    return ACC_TYPE(v_dot2_f32_f16(a, b, float(acc)));
+}
+
+#else
+
+ACC_TYPE dot_product(FLOAT_TYPEV4 a, FLOAT_TYPEV4 b, ACC_TYPE acc) {
+    return fma(ACC_TYPE(a.x), ACC_TYPE(b.x), fma(ACC_TYPE(a.y), ACC_TYPE(b.y),
+           fma(ACC_TYPE(a.z), ACC_TYPE(b.z), fma(ACC_TYPE(a.w), ACC_TYPE(b.w), acc))));
+}
+
+ACC_TYPE dot_product(FLOAT_TYPEV2 a, FLOAT_TYPEV2 b, ACC_TYPE acc) {
+    return fma(ACC_TYPE(a.x), ACC_TYPE(b.x), fma(ACC_TYPE(a.y), ACC_TYPE(b.y), acc));
+}
+
+#endif