[BE] Use `C10_UNUSED` (#111439)
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### <samp>🤖 Generated by Copilot at 21e87dc</samp>
> _We're sailing on the sea of code, with warnings to avoid_
> _We use the `C10_UNUSED` macro for variables unexploited_
> _We heave and ho and pull and push, and make the code more neat_
> _We sing this shanty as we go, to keep us in good spirits_
Pull Request resolved: https://github.com/pytorch/pytorch/pull/111439
Approved by: https://github.com/huydhn
