Improvements to device support

[asmeurer]

#59 added support for basic devices. Some improvements that could be made:

• Factor out the device checking logic (into a decorator?)
• Add devices that don't support certain dtypes (non-support for float64/complex128 being the most common)
• Allow the user to manually create custom "devices" with certain properties (probably via the flags API).
• Better testing for device support. This ideally should go in array-api-tests, but currently it doesn't test devices at all, so it would be useful to have some basic tests here.

[ogrisel]

I tried to run the scikit-learn tests with array-api-strict on the non-default device and I got some failures that do not happen with PyTorch and non-default device. So I suspect there is something fishy happening with the new devices in array-api-strict but I have not investigated the root cause yet.

scikit-learn/scikit-learn#30090

[betatim]

I'll take a look/investigate that failure further.