torch.func (pytorch 2.0) compatibility

[fzimmermann89]

Hi,

to be able to use torch.func.* (for example, torch.func.grad) with torchkbnufft, the torch.autograd.Functions in torchkbnufft/_autograd/interp.py would need an update to the newer calling signature.
See https://pytorch.org/docs/stable/notes/extending.func.html.

In particular, the forward should no longer take a ctx argument. Instead, it should return the output and all tensors etc. that need to be saved to ctx. And a third function, setup_context should save it in ctx.
Also, it would be nice to make use of vmap possible

As this would break compatibility for pytorch versions <2.0 (released before March 2023), this would either require a new torchkbnufft version and a bump in the required torch version, or an import-time switch depending on the pytorch version.

I would be willing to prepare a PR for either option.

[mmuckley]

Hello @fzimmermann89, this would be a very welcome contribution! Feel free to file a PR for this.

I think we can make a new version of torchkbnufft and require the newer PyTorch. I'm not a big fan of import switching.