[megatron] fix val_dataset (#4478)

Author: Jintao-Huang

swift/megatron/train/utils.py

@@ -19,7 +19,7 @@ def swift_datasets_provider(train_val_test_num_samples):
         step_batch_size = \
             args.micro_batch_size * data_parallel_size
         # To avoid errors caused by the validation set being insufficient to complete a single step.
-        if len(val_dataset) < step_batch_size:
+        if val_dataset is not None and len(val_dataset) < step_batch_size:
             val_dataset = None
         return train_dataset, val_dataset, None