XLNet tensor at wrong device issuse
```bash
File "env.xlnet/lib/python3.6/site-packages/pytorch_transformers/modeling_xlnet.py", line 397, in rel_shift
    x = torch.index_select(x, 1, torch.arange(klen))
RuntimeError: Expected object of backend CUDA but got backend CPU for argument #3 'index'
```

I meet this issue when using `pytorch-transformers==0.7.0` with multi-gpus, is quickfixed by `x = torch.index_select(x, 1, torch.arange(klen).to(x.device))`
This model was WIP. Fixed now.