Fix BERT example code for NSP and Multiple Choice (#3953)

Change the example code to use encode_plus since the token_type_id
wasn't being correctly set.

src/transformers/modeling_bert.py

@@ -1036,11 +1036,12 @@ class BertForNextSentencePrediction(BertPreTrainedModel):
         tokenizer = BertTokenizer.from_pretrained('bert-base-uncased')
         model = BertForNextSentencePrediction.from_pretrained('bert-base-uncased')
 
-        input_ids = torch.tensor(tokenizer.encode("Hello, my dog is cute", add_special_tokens=True)).unsqueeze(0)  # Batch size 1
-        outputs = model(input_ids)
-
-        seq_relationship_scores = outputs[0]
+        prompt = "In Italy, pizza served in formal settings, such as at a restaurant, is presented unsliced."
+        next_sentence = "The sky is blue due to the shorter wavelength of blue light."
+        encoding = tokenizer.encode_plus(prompt, next_sentence, return_tensors='pt')
 
+        loss, logits = model(**encoding, next_sentence_label=torch.LongTensor([1]))
+        assert logits[0, 0] < logits[0, 1] # next sentence was random
         """
 
         outputs = self.bert(
@@ -1191,7 +1192,7 @@ class BertForMultipleChoice(BertPreTrainedModel):
         r"""
         labels (:obj:`torch.LongTensor` of shape :obj:`(batch_size,)`, `optional`, defaults to :obj:`None`):
             Labels for computing the multiple choice classification loss.
-            Indices should be in ``[0, ..., num_choices]`` where `num_choices` is the size of the second dimension
+            Indices should be in ``[0, ..., num_choices-1]`` where `num_choices` is the size of the second dimension
             of the input tensors. (see `input_ids` above)
 
     Returns:
@@ -1221,14 +1222,17 @@ class BertForMultipleChoice(BertPreTrainedModel):
 
         tokenizer = BertTokenizer.from_pretrained('bert-base-uncased')
         model = BertForMultipleChoice.from_pretrained('bert-base-uncased')
-        choices = ["Hello, my dog is cute", "Hello, my cat is amazing"]
 
-        input_ids = torch.tensor([tokenizer.encode(s, add_special_tokens=True) for s in choices]).unsqueeze(0)  # Batch size 1, 2 choices
-        labels = torch.tensor(1).unsqueeze(0)  # Batch size 1
-        outputs = model(input_ids, labels=labels)
+        prompt = "In Italy, pizza served in formal settings, such as at a restaurant, is presented unsliced."
+        choice0 = "It is eaten with a fork and a knife."
+        choice1 = "It is eaten while held in the hand."
+        labels = torch.tensor(0) # choice0 is correct (according to Wikipedia ;))
 
-        loss, classification_scores = outputs[:2]
+        encoding = tokenizer.batch_encode_plus([[prompt, choice0], [prompt, choice1]], return_tensors='pt', pad_to_max_length=True)
+        outputs = model(**{k: v.unsqueeze(0) for k,v in encoding.items()}, labels=labels) # batch size is 1
 
+        # the linear classifier still needs to be trained
+        loss, logits = outputs[:2]
         """
         num_choices = input_ids.shape[1]
 

src/transformers/modeling_tf_bert.py

@@ -857,10 +857,13 @@ class TFBertForNextSentencePrediction(TFBertPreTrainedModel):
 
         tokenizer = BertTokenizer.from_pretrained('bert-base-uncased')
         model = TFBertForNextSentencePrediction.from_pretrained('bert-base-uncased')
-        input_ids = tf.constant(tokenizer.encode("Hello, my dog is cute", add_special_tokens=True))[None, :]  # Batch size 1
-        outputs = model(input_ids)
-        seq_relationship_scores = outputs[0]
 
+        prompt = "In Italy, pizza served in formal settings, such as at a restaurant, is presented unsliced."
+        next_sentence = "The sky is blue due to the shorter wavelength of blue light."
+        encoding = tokenizer.encode_plus(prompt, next_sentence, return_tensors='tf')
+
+        logits = model(encoding['input_ids'], token_type_ids=encoding['token_type_ids'])[0]
+        assert logits[0][0] < logits[0][1] # the next sentence was random
         """
         outputs = self.bert(inputs, **kwargs)
 
@@ -990,11 +993,15 @@ class TFBertForMultipleChoice(TFBertPreTrainedModel):
 
         tokenizer = BertTokenizer.from_pretrained('bert-base-uncased')
         model = TFBertForMultipleChoice.from_pretrained('bert-base-uncased')
-        choices = ["Hello, my dog is cute", "Hello, my cat is amazing"]
-        input_ids = tf.constant([tokenizer.encode(s) for s in choices])[None, :]  # Batch size 1, 2 choices
-        outputs = model(input_ids)
-        classification_scores = outputs[0]
 
+        prompt = "In Italy, pizza served in formal settings, such as at a restaurant, is presented unsliced."
+        choice0 = "It is eaten with a fork and a knife."
+        choice1 = "It is eaten while held in the hand."
+        encoding = tokenizer.batch_encode_plus([[prompt, choice0], [prompt, choice1]], return_tensors='tf', pad_to_max_length=True)
+
+        # linear classifier on the output is not yet trained
+        outputs = model(encoding['input_ids'][None, :])
+        logits = outputs[0]
         """
         if isinstance(inputs, (tuple, list)):
             input_ids = inputs[0]