Unify label args (#4722)

* Deprecate masked_lm_labels argument

* Apply to all models

* Better error message

src/transformers/modeling_ctrl.py

@@ -489,7 +489,7 @@ class CTRLLMHeadModel(CTRLPreTrainedModel):
         r"""
         labels (:obj:`torch.LongTensor` of shape :obj:`(batch_size, sequence_length)`, `optional`, defaults to :obj:`None`):
             Labels for language modeling.
-            Note that the labels **are shifted** inside the model, i.e. you can set ``lm_labels = input_ids``
+            Note that the labels **are shifted** inside the model, i.e. you can set ``labels = input_ids``
             Indices are selected in ``[-100, 0, ..., config.vocab_size]``
             All labels set to ``-100`` are ignored (masked), the loss is only
             computed for labels in ``[0, ..., config.vocab_size]``