Add assert to catch case when neither epochs or iters is set (#1368)

Author: aflah02

megatron/data/data_utils.py

@@ -516,6 +516,11 @@ def validate_train_epochs(neox_args):
 def build_train_valid_test_data_loaders(neox_args):
     """XXX"""
 
+    if not neox_args.train_epochs and not neox_args.train_iters:
+        raise ValueError(
+            "Must specify either train_epochs or train_iters"
+        )
+
     validate_train_epochs(neox_args)
 
     (train_dataloader, valid_dataloader, test_dataloader) = (None, None, None)