[PyTorch] add c10/util/FbcodeMaps.h (#96359)
Allow us to use folly maps in fbcode and std maps for compatibility in OSS, extending what static runtime is already doing.
Differential Revision: [D43926670](https://our.internmc.facebook.com/intern/diff/D43926670/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/96359
Approved by: https://github.com/ezyang
