Do not move some Subtensor to the GPU when this do not make sence.

When we do this on an inputs/shared var and that this is the only use
of it in the graph, we should only transfer to the GPU the small
computation.
If we keep the Subtensor on the CPU, keep the bubble active

fix gh-2327
Do the same on the new gpu back-end
Merge pull request #2329 from nouiz/gpu_subtensor

Do not move the GpuSubtensor to the GPU when this do not make sence