one context per device.

Only scan is left.

This unfortunately renders pycuda unusable without a significant
amount of work which I am not ready to do right now. Since we have C
code for this op I just removed the python code.

- Convert GpuKernelBase to use contexts.
- Start using explicit context for all the ops.

This may break some stuff outside of basic_ops.py since sometimes a
context will be required, but should be ok otherwise.