BartForSequenceClassification: fix num_labels, add test (#3110)

src/transformers/modeling_bart.py

@@ -1324,7 +1324,7 @@ class BartForSequenceClassification(PretrainedBartModel):
         # Prepend logits
         outputs = (logits,) + outputs[1:]  # Add hidden states and attention if they are here
         if labels is not None:  # prepend loss to output,
-            loss = F.cross_entropy(logits.view(-1, self.num_labels), labels.view(-1))
+            loss = F.cross_entropy(logits.view(-1, self.config.num_labels), labels.view(-1))
             outputs = (loss,) + outputs
 
         return outputs