[CUB] Investigate limiting execution space of `current_tuning_cc` and `current_policy`

The fact that `cub::detail::current_tuning_cc` and `cub::detail::current_policy` are usable from host code is a foot gun as it makes it easy to get into situations with diverging tunings accessed from host and device-code. It would be great if such a latent bug could be found before code-review. We should investigate if these functions could be made `__device__`-only to avoid such issues.

Trace the definitions and call sites of cub::detail::current_tuning_cc and cub::detail::current_policy in CUB, comparing their host and device usage. Check how restricting them to device code would affect existing callers and identify tests that should catch host-side access; done means the approach and compatibility impact are clear.

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