Summary:
Currently, AutoTimer outputs duration as a double with seconds as units.
This is limiting and I'm making it return std::chrono::duration. Users can specify the type with DurationType.

This is needed for me because the library that I am going to use inside the callback I pass in using chrono::duration and it feels hacky to go from duration -> double -> duration.

Reviewed By: yfeldblum

Differential Revision: D3506557

fbshipit-source-id: 0a5e9e16181bfac3a10df6a253716c0499cff5df