This is kind of tricky but not really. As the title says I am going to assume it makes no difference if it is a WI-FI model but if you buy from a carrier where you need a Data Plan so it would be 4G and WI-FI, Do they come unlocked?
Yes. Afaik in the US at least all iPads are sold unlocked. They aren't sold at subsidized prices so there's no reason for them to be locked.