Update on "[ET-VK] Fully De vectorise conv2d pw shader to improve perf."

This improves the performance of the conv2d pw shader by fully de-vectorizing it.
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.

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

[ghstack-poisoned]

Author: trivedivivek