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Add hints to device adapter scheduler

Kenneth Moreland requested to merge kmorel/vtk-m:hints into master

The DeviceAdapter provides an abstract interface to the accelerator devices worklets and other algorithms run on. As such, the programmer has less control about how the device launches each worklet. Each device adapter has its own configuration parameters and other ways to attempt to optimize how things are run, but these are always a universal set of options that are applied to everything run on the device. There is no way to specify launch parameters for a particular worklet.

To provide this information, VTK-m now supports Hints to the device adapter. The DeviceAdapterAlgorithm::Schedule method takes a templated argument that is of the type HintList. This object contains a template list of Hint types that provide suggestions on how to launch the parallel execution. The device adapter will pick out hints that pertain to it and adjust its launching accordingly.

These are called hints rather than, say, directives, because they don't force the device adapter to do anything. The device adapter is free to ignore any (and all) hints. The point is that the device adapter can take into account the information to try to optimize for itself.

A provided hint can be tied to specific device adapters. In this way, an worklet can further optimize itself. If multiple hints match a device adapter, the last one in the list will be selected.

The Worklet base now has an internal type named Hints that points to a HintList that is applied when the worklet is scheduled. Derived worklet classes can provide hints by simply defining their own Hints type.

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