Small inconsistency between exercises 17 and 18.

[lrpablo]

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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.