fixed BertForMultipleChoice model init and forward pass

pytorch_pretrained_bert/modeling.py

@@ -1034,13 +1034,13 @@ class BertForMultipleChoice(BertPreTrainedModel):
         self.num_choices = num_choices
         self.bert = BertModel(config)
         self.dropout = nn.Dropout(config.hidden_dropout_prob)
-        self.classifier = nn.Linear(config.hidden_size, 1)
+        self.classifier = nn.Linear(config.hidden_size, num_choices)
         self.apply(self.init_bert_weights)
 
     def forward(self, input_ids, token_type_ids=None, attention_mask=None, labels=None):
         flat_input_ids = input_ids.view(-1, input_ids.size(-1))
-        flat_token_type_ids = token_type_ids.view(-1, token_type_ids.size(-1))
-        flat_attention_mask = attention_mask.view(-1, attention_mask.size(-1))
+        flat_token_type_ids = token_type_ids.view(-1, token_type_ids.size(-1)) if token_type_ids is not None else None
+        flat_attention_mask = attention_mask.view(-1, attention_mask.size(-1)) if attention_mask is not None else None
         _, pooled_output = self.bert(flat_input_ids, flat_token_type_ids, flat_attention_mask, output_all_encoded_layers=False)
         pooled_output = self.dropout(pooled_output)
         logits = self.classifier(pooled_output)