support torch_device() and how it's used

[skeydan]

Not sure if we want this:

with (cuda_device(1)), {
    t1 <- torch_ones(c(2,2))
    t2 <- torch_randn(c(2,4))
    t1$mm(t2)
}

but we'd need something equivalent in function to

with torch.cuda.device(0):
    t1 = torch.ones((2,2))
    t2 = torch.randn((2,4))
    t1.mm(t2)

[dfalbel]

I explored this in the #127 but didnt have success... Will wait to see if I get answers in the forum.