Fix the log about length of valid_dataloader

tools/train.py

@@ -89,8 +89,10 @@ def main(config, device, logger, vdl_writer):
     # load pretrain model
     pre_best_model_dict = init_model(config, model, logger, optimizer)
 
-    logger.info('train dataloader has {} iters, valid dataloader has {} iters'.
-                format(len(train_dataloader), len(valid_dataloader)))
+    logger.info('train dataloader has {} iters'.format(len(train_dataloader)))
+    if valid_dataloader is not None:
+        logger.info('valid dataloader has {} iters'.format(
+            len(valid_dataloader)))
     # start train
     program.train(config, train_dataloader, valid_dataloader, device, model,
                   loss_class, optimizer, lr_scheduler, post_process_class,