Return correct Bart hidden state tensors (#8747)

* bart output hidden states upstream

* same w/ decoder

* add tests

* fix prophetnet

* fix gpt2 and ctrl

* fix fstm and skip test for reformer and longformer

* fix all models

Co-authored-by: Patrick von Platen <patrick.v.platen@gmail.com>

tests/test_modeling_longformer.py

@@ -328,6 +328,10 @@ class LongformerModelTest(ModelTesterMixin, unittest.TestCase):
         config_and_inputs = self.model_tester.prepare_config_and_inputs()
         self.model_tester.create_and_check_for_multiple_choice(*config_and_inputs)
 
+    def test_retain_grad_hidden_states_attentions(self):
+        # longformer cannot keep gradients in attentions or hidden states
+        return
+
 
 @require_torch
 @require_sentencepiece