macOS build failure due to missing metadata.json #753

Description

torch-cpu builds fail on macOS with

OSError: Failed to open "[...]/einops-torch-ext/metadata.json": No such file or directory

The reason is that no backend is passed to get_local_kernels, kernels (correctly) detects the Torch build as being Metal-enabled and then tries to open the Metal variant torch-metal, which does not exist in the output path.

Besides the underlying issue, the error is also a red herring - the issue isn't so much that metadata cannot be opened, but that the variant does not exist at all.

The solution is to add a backend argument to get_local_kernel and pass it inside the Nix derivation (plus improve detection, so that we get the correct error).

I have a fix for this already, but I haven't created a PR for it, since it conflicts with all the changes I'm working on for dependency handling, so I postponed it until the dependency handling PRs are merged.

Steps to reproduce

Build a torch-cpu variant on macOS.

Expected behavior

No error.

Environment

...

Additional context

No response