Support for Apple GPU builds

So far, apple has not been mentioned at all in #63, so I thought this deserves a new issue.

I'm also aware that the CI situation for apple is not great (especially for osx-arm), and that apple+GPU is even worse, but I'm thinking that in an ideal world, it should still be possible eventually.

Apple has some builds in a conda [channel](https://developer.apple.com/metal/tensorflow-plugin/), and here are their [install](https://developer.apple.com/metal/tensorflow-plugin/) instructions for the final product.

This leads to really contorted installation instructions in the wild (e.g. [here](https://developer.apple.com/metal/tensorflow-plugin/)), and consequently broken environments: https://github.com/conda-forge/tensorflow-feedstock/issues/154