By Tanner Garrity Don’t even think about serving Robert Pattinson “a piece of white fish with just a lemon on it.” For a little while, at least. The 35-year-old star of The Batman, which premieres this Friday, described his training regimen and diet in an interview with On Demand Entertainment, and he seems pretty excited to leave it in the rearview. “Training is a lot more fun when you kind of get good at it,” he said. “Once you pass over the six-week mark, then it’s alright. Then you feel bad if you’re not doing it. That’s kind of what everyone says … No matter how many times you realize: if…