Faster `best.pt` checkpoint saving (#9436)

ultralytics/engine/trainer.py

@@ -500,9 +500,9 @@ class BaseTrainer:
         # Save last and best
         torch.save(ckpt, self.last)
         if self.best_fitness == self.fitness:
-            torch.save(ckpt, self.best)
+            self.best.write_bytes(self.last.read_bytes())  # copy last.pt to best.pt
         if (self.save_period > 0) and (self.epoch > 0) and (self.epoch % self.save_period == 0):
-            torch.save(ckpt, self.wdir / f"epoch{self.epoch}.pt")
+            (self.wdir / f"epoch{self.epoch}.pt").write_bytes(self.last.read_bytes())  # copy last.pt to i.e. epoch3.pt
 
     @staticmethod
     def get_dataset(data):