Add "multi device" support

[betatim]

Having more than one device is useful during testing to allow you to find bugs related to how arrays on different devices are handled. Closes #56

With scikit-learn we run into the frustrating situation were contributors execute tests locally, they all pass but then see failures on the CI related to the fact that e.g. PyTorch has several devices and some things work on the CPU device but not on the CUDA/MPS device. However, if you have neither of those on your local machine you can't really test this upfront and to debug it you need to rely on the CI.

The idea of this PR is to add support for multiple devices to array-api-strict to make testing easier. The default device continues to be the CPU device and for arrays that use it nothing should change. However, you can now place an array on a different device with array_api_strict.Device("pony") (or some other string, each string is a new device). For arrays on a device that isn't the CPU device calls like np.asarray(some_strict_array) will raise an error. This mirrors how PyTorch treats arrays on the CPU and MPS device.

What isn't yet implemented in this PR is raising an error if you try to operate on arrays that are not on the same device.

I wanted to open this PR already now after just a short amount of effort to get feedback what people think about this before putting in the time to update all the tests, etc.

[lucascolley]

+1! Another question is what the default should be (technically Device("pony") is more strict), but probably better if we can keep the cpu default for backwards compatibility.

[betatim]

I think the CPU device should be the default. That way code that exists today should keep working and the only people who notice any changes are those who use the pony device.

[asmeurer]

This looks good so far. We need to make sure the semantics specified at https://data-apis.org/array-api/latest/design_topics/device_support.html#semantics are followed, namely, disallowing combining arrays from different devices, and making sure that if a function creates a new array based on an existing array that it uses the same device.

For tests, ideally this would be tested in array-api-tests, but right now device support is not tested at all there. If you just want to add some basic tests here for now, that is fien.

Finally, there is the devices inspection API. https://data-apis.org/array-api/latest/API_specification/generated/array_api.info.devices.html#array_api.info.devices We need to think about how that will work. One option would just be to create a small but fixed number of devices. Or we could add some flags to make it configurable https://data-apis.org/array-api-strict/api.html#array_api_strict.set_array_api_strict_flags