Add next sentence prediction loss computation (#8462)

* Add next sentence prediction loss computation

* Apply style

* Fix tests

* Add forgotten import

* Add forgotten import

* Use a new parameter

* Remove kwargs and use positional arguments

src/transformers/modeling_tf_outputs.py

@@ -307,6 +307,8 @@ class TFNextSentencePredictorOutput(ModelOutput):
     Base class for outputs of models predicting if two sentences are consecutive or not.
 
     Args:
+        loss (:obj:`tf.Tensor` of shape :obj:`(1,)`, `optional`, returned when :obj:`next_sentence_label` is provided):
+            Next sentence prediction loss.
         logits (:obj:`tf.Tensor` of shape :obj:`(batch_size, 2)`):
             Prediction scores of the next sequence prediction (classification) head (scores of True/False continuation
             before SoftMax).
@@ -323,6 +325,7 @@ class TFNextSentencePredictorOutput(ModelOutput):
             heads.
     """
 
+    loss: tf.Tensor = None
     logits: tf.Tensor = None
     hidden_states: Optional[Tuple[tf.Tensor]] = None
     attentions: Optional[Tuple[tf.Tensor]] = None