Display metatensor-torch torchscript classes correctly in docs

[frostedoyster]

Closes #20

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📚 Documentation preview 📚: https://metatensor-models--40.org.readthedocs.build/en/40/

[frostedoyster]

@Luthaf something is breaking, seemingly due to the fact that we're compiling slice and join, but I'm not sure. Any ideas?

[Luthaf]

Right =/

When setting METATENSOR_IMPORT_FOR_SPHINX=1, the definition of all classes is changed to something sphinx can parse and understand. But this means TorchScript can no longer parse/understand it! The simplest solution might be to do something like

if os.environ.get("METATENSOR_IMPORT_FOR_SPHINX", "0") == "1":
    compiled_slice = None
else:
    compiled_slice = torch.jit.script(metatensor.torch.slice)

[frostedoyster]

Perfect, thanks. I was also thinking of a similar hack