Make ProphetNetModel really compatible with EncoderDecoder (#9033)

* improve

* finish

* upload model

* fix lm head

* fix test

tests/test_modeling_prophetnet.py

@@ -38,6 +38,7 @@ if is_torch_available():
         ProphetNetModel,
         ProphetNetTokenizer,
     )
+    from transformers.modeling_outputs import BaseModelOutput
 
 
 class ProphetNetModelTester:
@@ -467,6 +468,31 @@ class ProphetNetModelTester:
             )
         )
 
+    def check_causal_lm_from_pretrained(
+        self, config, input_ids, decoder_input_ids, attention_mask, decoder_attention_mask, *args
+    ):
+        model = ProphetNetForConditionalGeneration(config).to(torch_device).eval()
+
+        with tempfile.TemporaryDirectory() as tmp_dirname:
+            model.save_pretrained(tmp_dirname)
+            decoder = ProphetNetForCausalLM.from_pretrained(tmp_dirname).to(torch_device)
+
+        encoder_hidden_states = model.prophetnet.encoder(input_ids).last_hidden_state
+
+        model_outputs = model(
+            encoder_outputs=BaseModelOutput(last_hidden_state=encoder_hidden_states),
+            decoder_input_ids=decoder_input_ids,
+        )
+        dec_outputs = decoder(encoder_hidden_states=encoder_hidden_states, input_ids=decoder_input_ids)
+
+        self.parent.assertTrue(
+            torch.allclose(
+                model_outputs.logits[0, :5],
+                dec_outputs.logits[0, :5],
+                atol=1e-3,
+            )
+        )
+
     def prepare_config_and_inputs_for_common(self):
         config_and_inputs = self.prepare_config_and_inputs()
         (
@@ -898,6 +924,10 @@ class ProphetNetModelTest(ModelTesterMixin, GenerationTesterMixin, unittest.Test
 
         self.assertFalse(config.add_cross_attention)
 
+    def test_causal_lm_from_pretrained(self):
+        config_and_inputs = self.model_tester.prepare_config_and_inputs()
+        self.model_tester.check_causal_lm_from_pretrained(*config_and_inputs)
+
     @unittest.skipIf(torch_device == "cpu", "Cant do half precision")
     def test_fp16_forward(self):
         config_and_inputs = self.model_tester.prepare_config_and_inputs()