[ET-VK] Using push constants for conv2d pw.

Pull Request resolved: https://github.com/pytorch/executorch/pull/7814

This diff is related to the use of push constants for convolutional pw (pointwise) in Executorch's Vulkan backend. This optimization improves performance and memory usage.
ghstack-source-id: 263238730
@exported-using-ghexport

Differential Revision: [D68400677](https://our.internmc.facebook.com/intern/diff/D68400677/)

Co-authored-by: Vivek Trivedi <5340687+trivedivivek@users.noreply.github.com>