Shorten namespace `api::utils` to `utils` (#4122)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/4122

New briefer convention since the new `vkapi` namespace introduced at the top of stack would otherwise be long and repetitive.

Most these functions don't deal with the Vulkan API anyway.
ghstack-source-id: 232367492
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Reviewed By: SS-JIA

Differential Revision: D59281540

fbshipit-source-id: 2c0f2e09e63c10ec8403b830f76e38bffe3672e0