Move `gen_vulkan_spv.py` out of `api/` (#4121)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/4121

It doesn't deal with the Vulkan API.
ghstack-source-id: 232367490
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Reviewed By: SS-JIA

Differential Revision: D59281544

fbshipit-source-id: 6b8dcc128aa971b83e46aee52a4b5e7894d1d443