fix tra when logdir is None

Author: Dong Zhou

qlib/contrib/model/pytorch_tra.py

@@ -352,7 +352,8 @@ def _fit(self, train_set, valid_set, test_set, evals_result, start_epoch=0, is_p
                     "model": copy.deepcopy(self.model.state_dict()),
                     "tra": copy.deepcopy(self.tra.state_dict()),
                 }
-                torch.save(best_params, self.logdir + "/model.bin")
+                if self.logdir is not None:
+                    torch.save(best_params, self.logdir + "/model.bin")
             else:
                 stop_rounds += 1
                 if stop_rounds >= self.early_stop: