Update namespace from at::native::vulkan to vkcompute (#2798)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2798

## Context

The previous change in the stack forks the Vulkan Compute API into the ExecuTorch directory. Now that there is no longer any dependency on ATen-Vulkan, the `at::native::vulkan` namespace is deprecated in favor of `vkcompute`.

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Reviewed By: jorgep31415

Differential Revision: D55608083

fbshipit-source-id: bfe7564359b19daf4356c54d9a2c0e967171d25f