Workaround NVCC parse failure in `cast_op` (#4893)

* Workaround NVCC parse failure in `cast_op`

There is a bug in some CUDA versions (observed in CUDA 12.1 and 11.7 w/ GCC 12.2),
that makes `cast_op` fail to compile:
  `cast.h:45:120: error: expected template-name before ‘<’ token`

Defining the nested type as an alias and using it allows this to work
without any change in semantics.

Fixes #4606

* style: pre-commit fixes

* Add comments to result_t referencing PR

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>