GPU Support

[under-Peter]

GPU support works with

• TRG forward
• TRG gradient
• CTMRG forward
• CTMRG gradient
• Variational Optimisation

In the #4 branch. There's an issue with CuArrays and Zygote documented here which currently prevents taking the gradient of either trg or ctmrg.
Once that issue's resolved, the rest should probably work.