OpenCL interoperability does not work on CUDA machine

On (Linux) CUDA machine, the backend returned by `af.get_backend()` remains at unified even if the backend is correctly set as OpenCL. `af.get_active_backend()` correctly returns OpenCL as the backend, but since the interoperability functions use different way to check the backend, an error is output (see https://github.com/arrayfire/arrayfire-python/blob/master/arrayfire/opencl.py#L107). Commenting all those instances fixes the issue. The (commented) tests reproduce this error, i.e. https://github.com/arrayfire/arrayfire-python/blob/master/tests/simple/interop.py#L82

Note that this wasn't an issue on a (Windows) AMD machine, where OpenCL is the default backend.

Start with the backend checks in arrayfire/opencl.py around line 107 and inspect the commented reproduction in tests/simple/interop.py around line 82. Run that interoperability test on a Linux CUDA machine and compare get_backend() with get_active_backend(); done means OpenCL interoperability succeeds without the reported error.

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