Fix c/p typo from my experiment code

pytorch_pretrained_bert/modeling_gpt2.py

@@ -619,7 +619,7 @@ class GPT2LMHeadModel(GPT2PreTrainedModel):
         if lm_labels is not None:
             # Shift so that tokens < n predict n
             shift_logits = lm_logits[:, :-1]
-            shift_labels = torch_batch[:, 1:]
+            shift_labels = lm_labels[:, 1:]
 
             # In tensorflow, it's [batch, d_0, d_1, ..., d_{r-1}, num_classes]
             # in pytorch, it's [batch, num_classes, d_0, d_1, ..., d_{r-1}]