[ET-VK][ez] Fix conv2d_pw shared memory buffer not having a constant size (#7734)

## Context

`conv2d_pw` was failing to compile on Mac due to the shared memory array not having a constant size.

Simply hardcode the workgroup size to fix.

Differential Revision: [D68331984](https://our.internmc.facebook.com/intern/diff/D68331984/)

ghstack-source-id: 261911463
Pull Request resolved: #7729

Co-authored-by: Stephen Jia <[email protected]>

Author: pytorchbot

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

@@ -33,7 +33,9 @@ ${layout_declare_ubo(8, "float", "out_min", "float", "out_max")}
 layout(local_size_x_id = 0, local_size_y_id = 1, local_size_z_id = 2) in;
 
 // shared memory to hold calculated positions, this would reduce register usage thus improving performance.
-shared ivec2 pos_shared[gl_WorkGroupSize.x * gl_WorkGroupSize.y * gl_WorkGroupSize.z * TILE_SIZE * TILE_SIZE];
+// 64 is the number of threads in the local wg
+$num_shared = 64 * TILE_SIZE * TILE_SIZE
+shared ivec2 pos_shared[${num_shared}];
 
 /*
  * Computes a 2D pointwise convolution of an NxN output tile. Calculating an