Fix calculate training time by summing all elapsed times instead the last one (#21291)

* fix: calculating training time by summing all differences instead of taking the last calculation

* test timings

* changelog

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Co-authored-by: itzhaks <itzhak.stern@mobileye.com>
Co-authored-by: Nicki Skafte Detlefsen <skaftenicki@gmail.com>
Co-authored-by: Justus Schock <12886177+justusschock@users.noreply.github.com>
Co-authored-by: Jirka Borovec <6035284+Borda@users.noreply.github.com>