add executor object

[SimeonEhrig]

The executor should be a container object, which contains all types and objects, which are required to launch a vikunja algorithm. His task is it to simplify the API.

Possible implementation for the basics:

executor.hpp

template<typename TAcc, typename TDev, typename TQueue>
class Executor {
public:
     using Acc = typename TAcc;

     TDev dev;
     TQueue queue;

     Executor(TDev dev, TQueue queue) dev(dev), queue(queue) {}
}

reduce.hpp

// fully compatible with alpaka types
template<Typename TAcc, typename TDev, typename TQueue>
reduce(TQueue queue, buffer input, int sum, Func functor){
   // actual reduce algorithm 
}

template<typename TAcc, typename TDev, typename TQueue>
reduce(Executor<TAcc, TDev, TQueue> executor, buffer input, int sum, Func functor){
  reduce<Executor<TAcc, TDev, TQueue>::Acc>(executor.queue, input, sum, functor);
}

main.cpp

// ...

reduce(executor, input, sum, reduce_func);
// instead
// reduce<Acc>(queue, input, sum, reduce_func);

The complete definition and properties are not fixed yet.
It should be still possible to use all possibilities of alpaka. The executor object should only simplify the default case.

[SimeonEhrig]

cc @j-stephan @psychocoderHPC

[SimeonEhrig]

TODO: view Sycl API

[j-stephan]

SYCL is a bit more low-level than vikunja (it doesn't provide host-side algorithms like reduce). What @psychocoderHPC was talking about today is the distinction SYCL makes between different objects. Memory is separate from queues and queues are separate from devices... they are not combined into god-like objects.

I like your proposed API. For reference, this is how the C++ executor proposal looks like:

executor auto ex = /* ... */;

std::reduce(std::execution::par.on(ex), /* first */, /* last */, /* init */, /* op */);
std::transform(std::execution::par.on(ex), /* first */, /* last */, /* out */, /* op */);

I believe we can achieve something similar with vikunja executors.