Eddie Redmayne is doing all he can to keep in shape for his role as the Emcee in “Cabaret” — which includes weekly bouts of “torture.” “It’s quite full-on,” he told The Post exclusively at the show’s opening Sunday. “There is this amazing man named Greg, who is kind of a genius, body-work human being who punishes me once a week,” the actor, 42, explained. “My wife [Hannah Bagshawe] thinks it’s massage, but it’s actually a kind of borderline torture. But it’s keeping me upright!” 6 Eddie Redmayne in “Cabaret” Marc Brenner 6 “My wife [Hannah Bagshawe] thinks it’s massage...