Make c10 dispatcher use boxed kernel function pointers (#16051)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16051
This changes the kernels stored in the c10 dispatcher from plain C function pointers to IValue-based KernelFunction*.
Note that KernelFunction is currently taking an `ArrayRef<IValue>` as arguments. A later diff will change that to it taking a `Stack*`.
Reviewed By: ezyang
Differential Revision: D13684518
fbshipit-source-id: 1fa54f60cec2e967b92a4a043d6e3ac1627ed991
