fix #993

pytorch_transformers/modeling_gpt2.py

@@ -683,9 +683,10 @@ class GPT2DoubleHeadsModel(GPT2PreTrainedModel):
 
         tokenizer = GPT2Tokenizer.from_pretrained('gpt2')
         model = GPT2DoubleHeadsModel.from_pretrained('gpt2')
-        choices = ["Hello, my dog is cute [CLS]", "Hello, my cat is cute [CLS]"]  # Assume you've added [CLS] to the vocabulary
+        tokenizer.add_special_tokens({'cls_token': '[CLS]'})  # Add a [CLS] to the vocabulary (we should train it also!)
+        choices = ["Hello, my dog is cute [CLS]", "Hello, my cat is cute [CLS]"]
         input_ids = torch.tensor([tokenizer.encode(s) for s in choices]).unsqueeze(0)  # Batch size 1, 2 choices
-        mc_token_ids = torch.tensor([-1, -1]).unsqueeze(0)  # Batch size 1
+        mc_token_ids = torch.tensor([input_ids.size(-1), input_ids.size(-1)]).unsqueeze(0)  # Batch size 1
         outputs = model(input_ids, mc_token_ids)
         lm_prediction_scores, mc_prediction_scores = outputs[:2]