Change model outputs types to self-document outputs (#5438)

* [WIP] Proposal for model outputs

* All Bert models

* Make CI green maybe?

* Fix ONNX test

* Isolate ModelOutput from pt and tf

* Formatting

* Add Electra models

* Auto-generate docstrings from outputs

* Add TF outputs

* Add some BERT models

* Revert TF side

* Remove last traces of TF changes

* Fail with a clear error message

* Add Albert and work through Bart

* Add CTRL and DistilBert

* Formatting

* Progress on Bart

* Renames and finish Bart

* Formatting

* Fix last test

* Add DPR

* Finish Electra and add FlauBERT

* Add GPT2

* Add Longformer

* Add MMBT

* Add MobileBert

* Add GPT

* Formatting

* Add Reformer

* Add Roberta

* Add T5

* Add Transformer XL

* Fix test

* Add XLM + fix XLMForTokenClassification

* Style + XLMRoberta

* Add XLNet

* Formatting

* Add doc of return_tuple arg

tests/test_modeling_t5.py

@@ -355,6 +355,7 @@ class T5ModelTest(ModelTesterMixin, unittest.TestCase):
         import tempfile
 
         config_and_inputs = self.model_tester.prepare_config_and_inputs()
+        config_and_inputs[0].return_tuple = True
         model = T5Model(config_and_inputs[0])
         with tempfile.TemporaryDirectory() as tmpdirname:
             torch.onnx.export(