Fix #21 and add export functionality.

[classner]

Hi folks!

Thanks for the great work with the torch plugin! I had a stab at fixing the problem in #21 , and also added the functionality for pdm torch export [...] so that requirement files can be generated.

In the process I noticed that the +cpu build identifier is unfortunately not universal (it doesn't work for Mac builds) and think it's okay to omit it for CPU builds, just let me know if that's not the case.

[polarathene]

I noticed that the +cpu build identifier is unfortunately not universal (it doesn't work for Mac builds)

+cpu local identifier is only assigned to x86_64 (AMD64) packages, PyTorch doesn't publish the aarch64 (ARM64) packages with that as there is no other GPU variants supported on that platform that PyTorch publishes (if you need CUDA compatible torch packages for aarch64 hardware like the Jetson, you'd need to install torch another way such as the containers nvidia publishes).