GPU integration

[benjaminvillalonga]

Connecting properly to TALSH interface so that GPU contractions are supported. Plan:

1. Get single contractions on GPU.
2. Pipeline GPU contractions.
3. Choose contraction orderings so that pipelines are as long as possible. This should be doable if contractions from two different patch expansions are alternated.
4. Choose what contractions go to CPU, single GPU, or multi-GPU. The single GPU ones are pipelined.