Update on "[ET-VK] Minor performance improvements to native layer norm."

This diff introduces minor performance improvements to the native layer norm function in the Vulkan backend of Executorch.

In this new approach:
The mean and variance values are calculated in 2 separate passes.
Shader is dispatched based on input texture size, and input texel is read and stored in shared memory.
Input stored in shard memory is then summed up using a reduce function.

This implementation better utilizes a GPUs parallel processing capabilities.

Differential Revision: [D72430290](https://our.internmc.facebook.com/intern/diff/D72430290/)

[ghstack-poisoned]

Author: trivedivivek