Good question. At a guess, I cannot imagine modern lithography and boost techniques allow manufacturers to get away with a single thermal diode anymore, so likely all modern gpus have strategically placed sensors all over the die.
That being said, I doubt that data is necessarily being reported to the programs we use to track GPU temps, although AMD might be doing something of the sort with their new "junction temperature" reporting in wattman.
Consumer side reporting software probably picks up an average of the various sensors or there is one primary sensor located near the middle of the die that returns that info.
Long and short of it is... I don't know.