add int to generated shader variants for copy_offset (#4033)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/4033

Also migrate to new layout codegen method + add int test cases in vulkan_compute_api_test.cpp

Reviewed By: copyrightly

Differential Revision: D58879830

fbshipit-source-id: 29a280f6dc734a5930b335784c0f2ba24e0d7efb