`grad(factory_op)` breaks with mixed cpu/cuda inputs

[bdhirsh]

This seems like a minor issue, but the following codes breaks:

        def foo(x):
            z = torch.zeros(1)  # factory func allocating on cpu
            z.copy_(x)  # cuda_tensor.copy_(cpu_tensor)
            return z.sum()

        x = torch.tensor(3.14, device='cuda')
        grad(foo))(x)

And fails with the error NotImplementedError: Cannot access storage of TensorWrapper.

I'm not sure whether the cpu_tensor.copy_(cuda_tensor) is actually valid, but it seems to work without the grad call.