[torchgen] Move Executorch custom ops logic into torchgen (#90099)
## Logic to handle custom ops
We generate files for custom ops, so that they can be registered into PyTorch.
Generated files:
* `Register{dispatch_key}CustomOps.cpp` (dispatch_key = CPU), it's basically the same as vanilla PyTorch `RegisterCPU.cpp`. The only difference is that we bind to native functions directly.
* `Register{dispatch_key}Stub.cpp` (dispatch_key = CPU), register placeholder kernels for custom ops. Only used when there's no custom op kernel available.
As an example:
```cpp
namespace {
at::Tensor & wrapper_out_unsqueeze_out(const at::Tensor & self, int64_t dim, at::Tensor & out) {
// No device check
// DeviceGuard omitted
return torch::executor::native::unsqueeze_out(self, dim, out);
}
} // anonymous namespace
TORCH_LIBRARY_IMPL(aten, CPU, m) {
m.impl("unsqueeze.out",
TORCH_FN(wrapper_out_unsqueeze_out));
}
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/90099
Approved by: https://github.com/ezyang
