[WIP] Use the new grad interface.

I don't know if argsort should have a grad defined, so I mark it as not implemented.

Don't merge, I'll add other stuff in that PR.

NEWS.txt:
- Make GpuCrossentropySoftmaxArgmax1HotWithBias and GpuCrossentropySoftmax1HotWithBiasDx work for bigger inputs (Frederic B., reported by Ryan Price)
* Add manual instruction for OpenBLAS on Ubuntu by (Jianri Li )
* Add extra debug info in verify_grad() (Frederic B.)
* pep8 (Frederic B.)
No news:
- make argsort use the new grad interface