[ET-VK] De vectorise positions in conv2d pw shader to improve perf.

Pull Request resolved: #11122

This improves the performance of the conv2d pw shader by de-vectorizing position storage.
The optimization involved replacing the `ivec3 pos` array with a plain `int pos` array to store the position values. The `x` and `y` coordinates are now stored in separate elements of the array instead of being stored together in an `ivec3`. This change allows for more efficient memory access and computation.
ghstack-source-id: 286439494
@exported-using-ghexport

Differential Revision: [D75335802](https://our.internmc.facebook.com/intern/diff/D75335802/)

Author: trivedivivek

backends/vulkan/runtime/graph/ops/glsl/conv2d_pw.glsl

@@ -64,20 +64,22 @@ void main() {
   // +--------+--------+
   // | pos[2] | pos[3] |
   // +--------+--------+
-  ivec3 pos[TILE_SIZE_X * TILE_SIZE_Y];
+  int pos[TILE_SIZE_X * TILE_SIZE_Y * 2];
   for (int y = 0, i = 0; y < TILE_SIZE_Y; ++y) {
     for (int x = 0; x < TILE_SIZE_X; ++x) {
-      pos[i] = ivec3(gpos.x * TILE_SIZE_X + x, gpos.y * TILE_SIZE_Y + y, gpos.z);
+      pos[i * 2] = gpos.x * TILE_SIZE_X + x;
+      pos[i * 2 + 1] = gpos.y * TILE_SIZE_Y + y;
       i++;
     }
   }
 
   // Compute the index of the input texture that needs to be loaded for each
   // output position. Note that negative indices can be produced indicating that
   // the top-left element is in a region added by padding.
-  ivec2 ipos[TILE_SIZE_X * TILE_SIZE_Y];
+  int ipos[TILE_SIZE_X * TILE_SIZE_Y * 2];
   for (int i = 0; i < TILE_SIZE_X * TILE_SIZE_Y; ++i) {
-    ipos[i] = pos[i].xy * stride - padding;
+    ipos[i * 2] = pos[i * 2] * stride.x - padding.x;
+    ipos[i * 2 + 1] = pos[i * 2 + 1] * stride.y - padding.y;
   }
 
   // Final output array where each element is a tensor value.
@@ -112,7 +114,7 @@ void main() {
     }
 
     for (int i = 0; i < TILE_SIZE_X * TILE_SIZE_Y; ++i) {
-      const vec4 in_tex = texelFetch(t_in, ivec3(ipos[i], z4), 0);
+      const vec4 in_tex = texelFetch(t_in, ivec3(ipos[i * 2], ipos[i * 2 + 1], z4), 0);
       // Load the input texel into an array
       float tex_values[4];
       tex_values[0] = in_tex.x;
@@ -163,8 +165,9 @@ void main() {
   }
 
   for (int i = 0; i < TILE_SIZE_X * TILE_SIZE_Y; ++i) {
-    if (all(lessThan(pos[i], out_limits.xyz))) {
-      imageStore(t_out, pos[i], op(vec4(sum[i * 4], sum[i * 4 + 1], sum[i * 4 + 2], sum[i * 4 + 3]), out_min, out_max));
+    const ivec3 pos_l = ivec3(pos[i * 2], pos[i * 2 + 1], gpos.z);
+    if (all(lessThan(pos_l, out_limits.xyz))) {
+      imageStore(t_out, pos_l, op(vec4(sum[i * 4], sum[i * 4 + 1], sum[i * 4 + 2], sum[i * 4 + 3]), out_min, out_max));
     }
   }
 }