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Draft: Merge fix on ITk seeder to reduce timing

Charline Rougier requested to merge crougier/athena:fix_timing_issue into 21.9

This MR is created to retrieve the timing performance of the default seeder of 21.9 while using the default tracking configuration of the seeder intended for master (in 21.9, for now).

Before the MR, the seeding timing of the default 21.9 seeder and the one intended for rmaster is found to be: timing_vs_Nseeds.pdf

One can notice that the timing of the seeder intended for master is worse than the default seeder. The problem is raised during the pixel seeding: timing_vs_Nseeds_pixel.pdf

This MR pinpoint the code responsible for this timing increase: an additional if loop is add in the pixel seed production, hence the issue. By replacing the few lines by what is done in the default seeder of 21.9, no segmentation fault is seen, and no impact on the performance is found (the exact same seed are found). The situation in the pixel seeding with this fix is now: timing_vs_Nseeds_pixel_FIX.pdf

Giving a total seeding time of: timing_vs_Nseeds_FIX.pdf

If this fix is accepted, it can be propagated in the master version as well 😄

(@lgagnon @pagessin)

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