Remove DDP destroy_process_group() on train end (#8935)

May resolve https://github.com/ultralytics/yolov5/issues/7307

train.py

@@ -527,9 +527,6 @@ def main(opt, callbacks=Callbacks()):
     # Train
     if not opt.evolve:
         train(opt.hyp, opt, device, callbacks)
-        if WORLD_SIZE > 1 and RANK == 0:
-            LOGGER.info('Destroying process group... ')
-            dist.destroy_process_group()
 
     # Evolve hyperparameters (optional)
     else: