Add C++ standard version check to top level headers (#40510)
Summary:
Remove `-std=c++14` flag from `utils.cmake`, since PyTorch C++ API can be invoked by any compiler compliant with C++14 standard or later
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40510
Differential Revision: D22253313
Pulled By: malfet
fbshipit-source-id: ff731525868b251c27928fc98b0724080ead9be2
