Create nGraph Context instead of CPU/GPU/NNP

[mbrookhart]

There are four main parts this refactor I see, but of course this might change: 1) change all of the NNP context stuff to be an nGraph context, 2) parse the nGraph context argument when creating the backend 3) replace the context of nodes we don’t fuse with something MXNet understands 4) make CI/validation work with the new context.

[cconvey]

AFAICT, our other upstreaming work is going to change the mechanism by which one mxnet backend delegates unsupported ops to MXnet's default CPU-based implementations. I'm going to pause work on this Issue until those other changes have been made, to avoid wasted effort.