[ET-VK] Replace Uniform buffers with push constants for copy op

Pull Request resolved: https://github.com/pytorch/executorch/pull/7267

This diff replaces uniform buffers with push constants for copy op in the Vulkan backend of Executorch. The changes include updating the GLSL code to use push constants instead of uniform buffers and updating the C++ code to pass the sizes as push constants to the shader.
ghstack-source-id: 259127151
@exported-using-ghexport

Differential Revision: [D66890851](https://our.internmc.facebook.com/intern/diff/D66890851/)

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