Fix unused variable error when OpenMP is disabled

[sitic]

Type

• Bug fix (non-breaking change which fixes an issue): Fixes Build error during build from source on Sonoma M1 Max #6514
• New feature (non-breaking change which adds functionality). Resolves #
• Breaking change (fix or feature that would cause existing functionality to not work as expected) Resolves #

Motivation and Context

error: unused variable 'kInnerThreads' [-Werror,-Wunused-variable]

type of errors occur when building without OpenMP on macOS Sonoma.

Checklist:

• I have run python util/check_style.py --apply to apply Open3D code style
  to my code.
• This PR changes Open3D behavior or adds new functionality.
  • Both C++ (Doxygen) and Python (Sphinx / Google style) documentation is
    updated accordingly.
  • I have added or updated C++ and / or Python unit tests OR included test
    results (e.g. screenshots or numbers) here.
• I will follow up and update the code if CI fails.
• For fork PRs, I have selected Allow edits from maintainers.

Description

Adds (void)kOuterThreads; style fixes to suppress the warnings.

[update-docs]

Thanks for submitting this pull request! The maintainers of this repository would appreciate if you could update the CHANGELOG.md based on your changes.

[saurabheights]

My C++ is very rusty, so just use these as suggestions (I could be wrong). Ideally, if a variable is not used, it should not be even declared. Furthermore, casting to void is too cryptic. There might be a better alternative where openmp related code is defined in #ifdef block - https://stackoverflow.com/questions/24488239/use-variable-from-cmake-in-c

Also, check for other OpenMP code in the library. There might be similar approach used.

[sitic]

Thank you for your suggestions, @saurabheights.

(void)variable; is a common pattern used to suppress the unused variable warnings, which are escalated to errors here. While it’s not the most elegant solution (in C++17 we could use [[maybe_unused]]), it's the most suitable approach in this case, in my opinion. I've added a code comment to clarify their purpose.

Indeed ideally unused variables should not be declared. Typically the number of threads will be computed in the OpenMP specific #pragma omp parallel line, however, in these two cases, they are separated out into new lines to manage line length. I don’t believe enclosing them in an #ifdef block is appropriate here, as it would make the code harder to read and more complex. They don’t do any meaningful work if OpenMP is disabled and will be optimized away, so it boils down to a preference in style.

[ssheorey]

Thanks @sitic we are planning to switch from C++14 to C++17 soon (some MSVC compatibility issues still remain), but this is a good workaround in the meantime.