Use some `if constexpr` in the code (#90483)
As PyTorch is C++17 project now. Replace `c10::guts::if_constexpr` with `if constexpr`
Deliberately delaying changes in headers until at least one nightly
cycle is complete.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/90483
Approved by: https://github.com/kit1980, https://github.com/Skylion007
