Move files using the Vulkan API to `vk_api/` (#4125)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/4125

and move from namespace `api` to `vkapi`. This gave me a major headache in the number of places to update.

This stack orgnaizes ET-VK neatly into three abstraction levels, both by folder and by namespace:
1. `namespace vkcompute` and `graph/`: for operator computation implementation and scheduling.
2. `namespace vkcompute::api` and `graph/api/`: for tensor objects (and other objects wrapping our VulkanBuffer/VulkanImage).
3. `namespace vkcompute::vkapi` and `graph/api/vk_api/`: for direct users of Vulkan API

Additionally, we have
- `namespace vkcompute::utils` and `graph/api/utils/`: for utils used by both namespace `api` and `vkapi`
ghstack-source-id: 232399401
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Reviewed By: SS-JIA

Differential Revision: D59281539

fbshipit-source-id: f6fe2ec3906187fdf86705796e6c477d7acfd648