Fix root_gpu crash with pytorch-lightning >= 1.8

AttributeError: `Trainer.root_gpu` was deprecated in
v1.6 and is no longer accessible as of v1.8. Please use
`Trainer.strategy.root_device.index` instead.

Support for Trainer.root_gpu was removed in:
Lightning-AI/pytorch-lightning#11994

train.py

@@ -126,7 +126,7 @@ def main():
   checkpoint_callback = pl.callbacks.ModelCheckpoint(save_last=args.save_last_network, every_n_epochs=args.network_save_period, save_top_k=-1)
   trainer = pl.Trainer.from_argparse_args(args, callbacks=[checkpoint_callback], logger=tb_logger)
 
-  main_device = trainer.root_device if trainer.root_gpu is None else 'cuda:' + str(trainer.root_gpu)
+  main_device = trainer.root_device if trainer.strategy.root_device.index is None else 'cuda:' + str(trainer.strategy.root_device.index)
 
   nnue.to(device=main_device)