Puree chiles, garlic, lemongrass, oil, ginger, cumin, sugar, and 1 tsp. salt in a blender or food processor until smooth. Score cut sides of eggplant in a cross-hatch pattern with the tip of a sharp knife. Coat each eggplant liberally with curry paste; set aside any remaining paste. Let eggplants sit at least 10 minutes and up to 1 hour. Prepare a grill for medium-high heat. Lightly season eggplants with salt and grill, turning occasionally, until browned and tender, about 5 minutes per side. Serve with reserved curry paste. Marinade can be made 3 days ahead. Cover and chill.