Max-digits of digit summing
I understand that you are asking from a parallel point of view, but I am unable to provide feedback on that, as I have never worked with this system before. But from my point of view, creating a parallel solution is has been done premature.

Prior to creating a parallel solution to speed up your solution, you should improve the efficiency of the algorithm. At present you consider all numbers in the range of [1, 10N), whereas you only need to evaluate numbers where the number of digits in the number is equal to N. In which case you only need to evaluate numbers in the range of [10N-1, 10N).

You could further improve this via considering permutations and combinations. This would allow you to calculate all the different combinations of the digits for the given range. Once you have these you can then check which of the combinations digits sum to the required total. 

If X combinations sum to a given total then you can quickly calculate how many numbers within the require range have N digits and sum to the goal. By computing X * (N Perm N) where N is the number of digits and where permutations are calculated without replacement. Representing just the N digits in the solution.

At this point you would be using a parallel solution to calculate the combination hits, but you may find the bottle neck of data transfer to a GPU causes the parallel solution to perform slower than the non-GPU enabled solution.