[ET-VK] Tuning native layer norm local workgroup size to improve thread occupancy during reduce.

This diff is tuning the local workgroup size of the native layer norm operation in Vulkan backend of Executorch to improve thread occupancy during the reduce phase.

Differential Revision: [D72581293](https://our.internmc.facebook.com/intern/diff/D72581293/)

[ghstack-poisoned]

Author: trivedivivek

backends/vulkan/runtime/graph/ops/impl/NativeLayerNorm.cpp

@@ -84,7 +84,18 @@ void add_native_layer_norm_node(
   std::vector<int64_t> in_sizes = t_input->sizes();
 
   utils::uvec3 global_size = t_out->logical_limits();
-  utils::uvec3 local_size = graph.create_local_wg_size(global_size);
+  utils::uvec3 local_size;
+
+  // Since the shader sets shared memory scale factor > 1, if dispatch is
+  // greater than maximum WG size. Setting WG size in X axis to max WG size,
+  // would allow best thread utilization.
+  if (global_size[0] > 64) {
+    local_size = {64, 1, 1};
+  } else {
+    // If thread size in X axis is smaller or equal to maximum WG size, we can
+    // let the function decide the best WG size.
+    local_size = graph.create_local_wg_size(global_size);
+  }
 
   std::string kernel_name("native_layer_norm");
   kernel_name.reserve(kShaderNameReserve);