Native automatic mixed precision for torchbearer

[mcarilli]

Native automatic mixed precision support (torch.cuda.amp) is now in master:
https://pytorch.org/docs/master/amp.html
https://pytorch.org/docs/master/notes/amp_examples.html

Not sure if you ever tried Nvidia's (our) experimental Apex Amp, but I know it has many pain points (extension builds, forward/backward compatibilty, DataParallel support, flaky checkpointing, i don’t even know if it can be hacked to handle double backward/gradient penalty, others…). torch.cuda.amp fixes all these, the interface is more flexible and intuitive, and the tighter integration with pytorch brings more future optimizations into scope.

I think the torch.cuda.amp API is a good fit for a higher-level library because its style is more functional (as in, it doesn't statefully alter anything outside itself). The necessary torch.cuda.amp calls don't have silent/weird effects elsewhere.

If you want to talk about adding torch.cuda.amp to torchbearer, with an eye towards it becoming the future-proof source of mixed precision, message me on Pytorch slack anytime (or ask me for invites if you're not signed up). I'll check this issue periodically but I'm on Pytorch slack a greater fraction of the time than I care to admit.