Move cuda/runtime/shim to cuda/runtime for better usibility.

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This PR was created by the merge bot to help merge the original PR into the main branch.
ghstack PR number: #14879 by @Gasoonjia
^ Please use this as the source of truth for the PR details, comments, and reviews
ghstack PR base: https://github.com/pytorch/executorch/tree/gh/gasoonjia/44/base
ghstack PR head: https://github.com/pytorch/executorch/tree/gh/gasoonjia/44/head
Merge bot PR base: https://github.com/pytorch/executorch/tree/main
Merge bot PR head: https://github.com/pytorch/executorch/tree/gh/gasoonjia/44/orig
Differential Revision: D84122729
@diff-train-skip-merge

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