Dated graphic in 1.2-automatic-differentiation.ipynb

<img width="179" alt="image" src="https://user-images.githubusercontent.com/11933538/45076463-b593d380-b0ea-11e8-8367-24d9ee111235.png">

Pull request [Fix attribute names (creator -> grad_fn)](https://github.com/pytorch/tutorials/pull/91/files) changed `creator` to `grad_fn`. 