Add compile-time promote_types template

Summary: Now types can be promoted at compile-time. (I had to fix promoteTypes' lack of gating for BFloat16; I believe that would have caused a buffer overflow?)

Reviewed By: kimishpatel, manuelcandales

Differential Revision: D56643045

fbshipit-source-id: cd522e50f59ce838bba06c796e47a1d16ac55b22