When I try to switch backend using af.set_backend('cuda'), I got error like:

It seems that I do not have cuda devices as candiated backend although I have 8 gpus in fact, and the output of af.info()
In [2]: af.info()
ArrayFire v3.6.2 (CPU, 64-bit Linux, build dc38ef1)
[0] Intel: Intel(R) Xeon(R) CPU E5-2699 v4 @ 2.20GHz, 257588 MB, Max threads(44)
and af.get_backend()
In [2]: af.get_backend()
Out[2]: 'unified'
So does I missing something when installing the package?