Make `as_array()` generic

[sebffischer]

as_array() is defined both in torch and in anvil (via the tengen dependency). It would be great if torch also made as_array() generic so we don't overwrite the functions.

[dfalbel]

Happy to do it as soon as tendon is on CRAN!

[sebffischer]

I think it's even possible without depending on tengen, we can just define the as_array generic in torch as well and implement as_array.torch_tensor. At least to my understanding this should be fine with S3, even when torch and tengen are both loaded.