Whenever possible, use function pointers rather than std::function to represent Operation's. (#26560)
Summary:
This takes a lot of pressure off of the C++ typechecker as well as generating much more
efficient and smaller code. In my not-super-rigorous testing, compile time for
register_prim_ops.cpp went from 68s to 35s, and the size of libtorch went from 72MB to 70MB.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/26560
Differential Revision: D17507305
fbshipit-source-id: 8bbd2c08304739432efda96da71f0fa80eb7668b
