add fmt, take 7 (#37356)
Summary:
fmt is a formatting library for C++. It has several properties that make it nice
for inclusion in PyTorch:
- Widely used
- Basically copies how Python does it
- Support for all the compilers and platforms we care about
- Standards track (C++20)
- Small code size
- Header only
This PR includes it as a submodule and sets up the build.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/37356
Differential Revision: D21262619
Pulled By: suo
fbshipit-source-id: 1d9a1a5ed08a634213748e7b02fc718ef8dac4c9
