As TF Cornerstone tees up its office-to-residential conversion of 135 E. 57th St., shoppers have hounded us about what would replace the Saks OFF Fifth store, which will close after the holidays. Wonder no more: the developers signed a lease with Chelsea Piers Fitness for 47,000 square feet. The  brand also has outposts at TFC’s at 33 Bond St. and 595 Dean St. in Brooklyn and at SL Green’s One Madison Avenue. The project will yield 350 apartments, with major construction to start early next