Better names for WG_SIZE/NUM_TD_PER_THREAD

[tom91136]

We should eventually consolidate all the different terms. Maybe keep only two, so something like:

• UNROLL_ITER for the static vector/loop unroll thing. Should be available in all impl.
• WG_SIZE only exists for implementations with nd_range, such as CL, CUDA, and SYCL And no more NUM_TD_PER_THREAD or ppWI