clean up bert next_data (#13650)

train iter was designed to never stop for both real and fake data

examples/mlperf/model_train.py

@@ -1142,12 +1142,7 @@ def train_bert():
           train_data["masked_lm_ids"], train_data["masked_lm_weights"], train_data["next_sentence_labels"])
 
         pt = time.perf_counter()
-
-        try:
-          next_data = next(train_it)
-        except StopIteration:
-          next_data = None
-
+        next_data = next(train_it)
         dt = time.perf_counter()
 
         device_str = parameters[0].device if isinstance(parameters[0].device, str) else f"{parameters[0].device[0]} * {len(parameters[0].device)}"