Small inconsistency between exercises 17 and 18.

[lrpablo]

The local range is defined as 'auto localRange = sycl::range(1, 32);' in exercise 17 and as 'auto localRange = sycl::range(8, 8);' in exercise 18. Given that the idea from exercises 16 to 18 is to see how performance can be increased by using different mechanisms, I think a change in the size of the range is not a good idea.