add back NNPACK in PyTorch (#15924)

add back NNPACK in PyTorch (#15924)

Summary:
This tests the water for adding back NNPACK in PyTorch, it's a lot better than the fallback THNN versions.

In #6151, we (ezyang and soumith) removed NNPACK support from PyTorch. Of course Maratyszcza might have advice, too. (Or an opinion on the CMake changes.)

The only functional changes are to use NNPack more aggressively on mobile and a .contiguous() to match NNPack's assumption (I stumbled over that while using NNPack for style transfer.)
The CMake changes try to use the NNPack we already have in git.

In terms of lines of code this is a large part of the diff of https://lernapparat.de/pytorch-jit-android/ . As far as I can tell, we don't have MKLDNN on mobile and the native THNN implementation are prohibitively expensive in terms of both CPU and memory.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15924

Differential Revision: D13709576

Pulled By: ezyang

fbshipit-source-id: f2e287739909451c173abf046588209a7450ca2c