Change period into every_n_epochs

it was changed in some recent version of pytorch-lightning
relevant docs https://pytorch-lightning.readthedocs.io/en/stable/api/pytorch_lightning.callbacks.model_checkpoint.html

Author: vondele

train.py

@@ -81,7 +81,7 @@ def main():
   print('Using log dir {}'.format(logdir), flush=True)
 
   tb_logger = pl_loggers.TensorBoardLogger(logdir)
-  checkpoint_callback = pl.callbacks.ModelCheckpoint(save_last=True, period=20, save_top_k=-1)
+  checkpoint_callback = pl.callbacks.ModelCheckpoint(save_last=True, every_n_epochs=20, 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)