bugfix: ensure that at::{dispatch_key}:: API gets external linkage (#58569)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/58569
This should allow external C++ files that aren't compiled into `libtorch.so`/`libtorch_cpu.so` (including all of fbcode) to use fast path functions like `at::cpu::add()`, which skip the dispatcher.
So, after spending way too much time trying to figure out why I was getting linker errors when calling `at::meta::{op}` and `at::cpu::{op}` from C++ test files, I realized that we're not including the header files for C++ for the namespaced operator definitions. I.e. `RegisterCPU.cpp`, which provides definitions for the `at::cpu::{op}` fast path functions, wasn't including the `CPUFunctions.h` header.
Why that breaks stuff: the `CPUFunctions.h` header file is what marks each function with the `TORCH_API` macro, so without including it, when we build `libtorch.so` and `libtorch_cpu.so`, the compiler will look at the definition in `RegisterCPU.cpp`, not see a `TORCH_API`, and decide that the function should get internal linkage.
An alternative would be to directly mark the function definitions in `RegisterCPU.cpp` with `TORCH_API`, but this seemed cleaner.
Test Plan: Imported from OSS
Reviewed By: pbelevich
Differential Revision: D28711300
Pulled By: bdhirsh
fbshipit-source-id: 535f245c20e977ff566d6da0757b3cefa137040b
