fixed TransfoXLLMHeadModel documentation (#3661)

Co-authored-by: TevenLeScao <teven.lescao@gmail.com>

src/transformers/modeling_transfo_xl.py

@@ -859,7 +859,7 @@ class TransfoXLLMHeadModel(TransfoXLPreTrainedModel):
 
     Return:
         :obj:`tuple(torch.FloatTensor)` comprising various elements depending on the configuration (:class:`~transformers.TransfoXLConfig`) and inputs:
-        loss (:obj:`torch.FloatTensor` of shape `(1,)`, `optional`, returned when ``labels`` is provided)
+        loss (:obj:`torch.FloatTensor` of shape `(batch_size, sequence_length)`, `optional`, returned when ``labels`` is provided)
             Language modeling loss.
         prediction_scores (:obj:`torch.FloatTensor` of shape :obj:`(batch_size, sequence_length, config.vocab_size)`):
             Prediction scores of the language modeling head (scores for each vocabulary token before SoftMax).