[ET-VK] Minor improvement to q_linear op shader.

This diff contains a minor improvement to the q_linear op shader in the Vulkan backend for Executorch. The code changes in the q_8w_linear.glsl file include a change in position parameter from a 3-element u16vec3 to a 2-element u16vec2.

Differential Revision: [D68113154](https://our.internmc.facebook.com/intern/diff/D68113154/)

ghstack-source-id: 261906021
Pull Request resolved: #7728

Author: trivedivivek

backends/vulkan/runtime/graph/ops/glsl/q_8w_linear.glsl

@@ -92,7 +92,7 @@ void main() {
 
 #extension GL_EXT_shader_explicit_arithmetic_types_int16 : require
 
-VEC4_T q_8w_linear(const u16vec3 out_pos, const uint16_t K) {
+VEC4_T q_8w_linear(const u16vec2 out_pos, const uint16_t K) {
   const uint16_t qmat2_pos_y = out_pos.x * uint16_t(4);
 
   VEC4_T outtex = VEC4_T(0);
@@ -101,7 +101,7 @@ VEC4_T q_8w_linear(const u16vec3 out_pos, const uint16_t K) {
   const VEC4_T scales = load_texel(t_scales, scales_pos);
 
   for (uint16_t i = uint16_t(0), x = uint16_t(0); i < K; i += uint16_t(4), x++) {
-    const VEC4_T mat1_tex = load_texel(t_mat1, u16vec3(x, out_pos.yz));
+    const VEC4_T mat1_tex = load_texel(t_mat1, u16vec3(x, out_pos.y, 0));
     const VEC4_T sums = VEC4_T(
         dot(mat1_tex, load_texel(t_qmat2, u16vec3(x, qmat2_pos_y, 0))),
         dot(mat1_tex, load_texel(t_qmat2, u16vec3(x, qmat2_pos_y + uint16_t(1), 0))),
@@ -117,16 +117,15 @@ VEC4_T q_8w_linear(const u16vec3 out_pos, const uint16_t K) {
 }
 
 void main() {
-  const u16vec3 out_pos = u16vec3(
+  const u16vec2 out_pos = u16vec2(
     gl_GlobalInvocationID.x / out_limits.y,
-    gl_GlobalInvocationID.x % out_limits.y,
-    0);
+    gl_GlobalInvocationID.x % out_limits.y);
   if (out_pos.x >= out_limits.x) {
     return;
   }
 
   VEC4_T outtex = q_8w_linear(out_pos, uint16_t(mat1_sizes.x));
-  write_texel(t_out, out_pos, outtex);
+  write_texel(t_out, u16vec3(out_pos, 0), outtex);
 }
 
 #endif