default in __init__s for classification BERT models (#650)

pytorch_pretrained_bert/modeling.py

@@ -980,7 +980,7 @@ class BertForSequenceClassification(BertPreTrainedModel):
     logits = model(input_ids, token_type_ids, input_mask)
     ```
     """
-    def __init__(self, config, num_labels):
+    def __init__(self, config, num_labels=2):
         super(BertForSequenceClassification, self).__init__(config)
         self.num_labels = num_labels
         self.bert = BertModel(config)
@@ -1045,7 +1045,7 @@ class BertForMultipleChoice(BertPreTrainedModel):
     logits = model(input_ids, token_type_ids, input_mask)
     ```
     """
-    def __init__(self, config, num_choices):
+    def __init__(self, config, num_choices=2):
         super(BertForMultipleChoice, self).__init__(config)
         self.num_choices = num_choices
         self.bert = BertModel(config)
@@ -1115,7 +1115,7 @@ class BertForTokenClassification(BertPreTrainedModel):
     logits = model(input_ids, token_type_ids, input_mask)
     ```
     """
-    def __init__(self, config, num_labels):
+    def __init__(self, config, num_labels=2):
         super(BertForTokenClassification, self).__init__(config)
         self.num_labels = num_labels
         self.bert = BertModel(config)