Allow text generation for ProphetNetForCausalLM (#9707)

* Moved ProphetNetForCausalLM's parent initialization after config update

* Added unit tests for generation for ProphetNetForCausalLM

tests/test_modeling_prophetnet.py

@@ -302,6 +302,24 @@ class ProphetNetModelTester:
         output_with_past_cache = model.generate(input_ids[:1], num_beams=2, max_length=5, do_sample=True)
         self.parent.assertTrue(torch.all(output_with_past_cache == output_without_past_cache))
 
+    def create_and_check_decoder_generate_with_past_key_value_states(
+        self,
+        config,
+        input_ids,
+        decoder_input_ids,
+        attention_mask,
+        decoder_attention_mask,
+        lm_labels,
+    ):
+        model = ProphetNetForCausalLM(config=config).to(torch_device).eval()
+        torch.manual_seed(0)
+        output_without_past_cache = model.generate(
+            input_ids[:1], num_beams=2, max_length=10, do_sample=True, use_cache=False
+        )
+        torch.manual_seed(0)
+        output_with_past_cache = model.generate(input_ids[:1], num_beams=2, max_length=10, do_sample=True)
+        self.parent.assertTrue(torch.all(output_with_past_cache == output_without_past_cache))
+
     def create_and_check_model_fp16_forward(
         self,
         config,
@@ -911,6 +929,10 @@ class ProphetNetModelTest(ModelTesterMixin, GenerationTesterMixin, unittest.Test
         config_and_inputs = self.model_tester.prepare_config_and_inputs()
         self.model_tester.create_and_check_generate_with_past_key_value_states(*config_and_inputs)
 
+    def test_encoder_decoder_model_generate(self):
+        config_and_inputs = self.model_tester.prepare_config_and_inputs()
+        self.model_tester.create_and_check_decoder_generate_with_past_key_value_states(*config_and_inputs)
+
     def test_attn_mask_model(self):
         config_and_inputs = self.model_tester.prepare_config_and_inputs()
         self.model_tester.check_model_with_attn_mask(*config_and_inputs)