fix GPT2DoubleHeadModel docstring

pytorch_transformers/modeling_gpt2.py

@@ -568,8 +568,12 @@ class GPT2LMHeadModel(GPT2PreTrainedModel):
 
     Examples::
 
+        import torch
+        from pytorch_transformers import GPT2Tokenizer, GPT2LMHeadModel
+
         tokenizer = GPT2Tokenizer.from_pretrained('gpt2')
         model = GPT2LMHeadModel.from_pretrained('gpt2')
+
         input_ids = torch.tensor(tokenizer.encode("Hello, my dog is cute")).unsqueeze(0)  # Batch size 1
         outputs = model(input_ids, labels=input_ids)
         loss, logits = outputs[:2]
@@ -684,12 +688,20 @@ class GPT2DoubleHeadsModel(GPT2PreTrainedModel):
         
         tokenizer = GPT2Tokenizer.from_pretrained('gpt2')
         model = GPT2DoubleHeadsModel.from_pretrained('gpt2')
-        tokenizer.add_special_tokens({'cls_token': '[CLS]'})  # Add a [CLS] to the vocabulary (we should train it also!)
-        model.resize_token_embeddings(len(tokenizer))  # Update the model embeddings to the new vocabulary size (add a vector at the end)
+        
+        # Add a [CLS] to the vocabulary (we should train it also!)
+        tokenizer.add_special_tokens({'cls_token': '[CLS]'})
+        model.resize_token_embeddings(len(tokenizer))  # Update the model embeddings with the new vocabulary size
+        print(tokenizer.cls_token_id, len(tokenizer))  # The newly token the last token of the vocabulary
+        
         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([input_ids.size(-1)])  # Batch size 1
-        outputs = model(input_ids, mc_token_ids)
+        encoded_choices = [tokenizer.encode(s) for s in choices]
+        cls_token_location = [tokens.index(tokenizer.cls_token_id) for tokens in encoded_choices]
+
+        input_ids = torch.tensor(encoded_choices).unsqueeze(0)  # Batch size: 1, number of choices: 2
+        mc_token_ids = torch.tensor([cls_token_location])  # Batch size: 1
+
+        outputs = model(input_ids, mc_token_ids=mc_token_ids)
         lm_prediction_scores, mc_prediction_scores = outputs[:2]
 
     """