Update on "[ET-VK] Using shared memory offsetting in conv2d pw and saving ivec3 pos instead of ivec2 to improve performance."

This diff changes conv2d pw op shader to offset shared memory based on thread local index to improve performance. Change also saves pos as ivec3 pos instead of ivec2.

Differential Revision: [D68400786](https://our.internmc.facebook.com/intern/diff/D68400786/)

[ghstack-poisoned]

Author: trivedivivek