[ET-VK] De vectorizing sum and moving bias application to the end in conv 2d op to improve performance.

This diff optimizes the conv 2d op in the Vulkan runtime by de-vectorizing the sum and moving the bias application to the end.

Differential Revision: [D75551846](https://our.internmc.facebook.com/intern/diff/D75551846/)

[ghstack-poisoned]

Author: trivedivivek

backends/vulkan/runtime/graph/ops/glsl/conv2d.glsl

@@ -12,6 +12,8 @@
 
 #define VEC4_T ${texel_type(DTYPE)}
 
+#define T ${texel_component_type(DTYPE)}
+
 #define op(X, A, B) ${OPERATOR}
 
 #include "indexing_utils.h"
@@ -72,13 +74,24 @@ void main() {
   kstart.y += pos.z * kernel_size.y;
 
   // Perform the convolution by iterating over the overlay region.
-  VEC4_T sum = texelFetch(t_bias, ivec2(pos.z, 0), 0);
+  T sum[4];
+  sum[0] = T(0);
+  sum[1] = T(0);
+  sum[2] = T(0);
+  sum[3] = T(0);
+
   const int ic4 = in_group_size / 4;
   for (int z4 = 0; z4 < ic4; ++z4, kstart.x += kernel_size.x * 4) {
     for (int y = start.y, ky = kstart.y; y < end.y; y += dilation.y, ++ky) {
       for (int x = start.x, kx = kstart.x; x < end.x; x += dilation.x, kx += 4) {
-        const VEC4_T in_texel = texelFetch(t_in, ivec3(x, y, z4), 0);
-        const ivec4 kxs = kx + ivec4(0, 1, 2, 3);
+        const VEC4_T in_texel_v = texelFetch(t_in, ivec3(x, y, z4), 0);
+
+        T in_texel[4];
+        in_texel[0] = in_texel_v.x;
+        in_texel[1] = in_texel_v.y;
+        in_texel[2] = in_texel_v.z;
+        in_texel[3] = in_texel_v.w;
+
 
         // To explain the calculation below, the contents of in_texel and the
         // group of 4 texels loaded from t_kernel are shown:
@@ -112,13 +125,27 @@ void main() {
         //
         // which is expressed in the following statements.
 
-        sum = fma(in_texel.xxxx, texelFetch(t_kernel, ivec2(kxs.x, ky), 0), sum);
-        sum = fma(in_texel.yyyy, texelFetch(t_kernel, ivec2(kxs.y, ky), 0), sum);
-        sum = fma(in_texel.zzzz, texelFetch(t_kernel, ivec2(kxs.z, ky), 0), sum);
-        sum = fma(in_texel.wwww, texelFetch(t_kernel, ivec2(kxs.w, ky), 0), sum);
+        T k_tex_arr[16];
+        for (int kc = 0; kc < 4; kc++) {
+          const VEC4_T k_tex = texelFetch(t_kernel, ivec2(kx + kc, ky), 0);
+          k_tex_arr[kc * 4 + 0] = k_tex.x;
+          k_tex_arr[kc * 4 + 1] = k_tex.y;
+          k_tex_arr[kc * 4 + 2] = k_tex.z;
+          k_tex_arr[kc * 4 + 3] = k_tex.w;
+        }
+
+        for (int sc = 0; sc < 4; sc++) {
+          sum[0] = fma(in_texel[sc], k_tex_arr[sc * 4 + 0], sum[0]);
+          sum[1] = fma(in_texel[sc], k_tex_arr[sc * 4 + 1], sum[1]);
+          sum[2] = fma(in_texel[sc], k_tex_arr[sc * 4 + 2], sum[2]);
+          sum[3] = fma(in_texel[sc], k_tex_arr[sc * 4 + 3], sum[3]);
+        }
       }
     }
   }
 
-  imageStore(t_out, pos, op(sum, out_min, out_max));
+  const VEC4_T bias = texelFetch(t_bias, ivec2(pos.z, 0), 0);
+  const VEC4_T out_sum = VEC4_T(sum[0], sum[1], sum[2], sum[3]) + bias;
+
+  imageStore(t_out, pos, op(out_sum, out_min, out_max));
 }