[ProphetNet] Bart-like Refactor (#10501)

* first step to refactor

* make all fast tests pass

* make all slow tests pass

* save intermediate

* correct cache

* finish PR

* make fp16 work

tests/test_modeling_prophetnet.py

@@ -243,7 +243,7 @@ class ProphetNetModelTester:
         # There should be `num_layers` key value embeddings stored in decoder_past
         self.parent.assertEqual(len(decoder_past), config.num_decoder_layers)
         # There should be a self attn key, a self attn value, a cross attn key and a cross attn value stored in each decoder_past tuple
-        self.parent.assertEqual(len(decoder_past[0]), 2)  # cross-attention + uni-directional self-attention
+        self.parent.assertEqual(len(decoder_past[0]), 4)  # cross-attention + uni-directional self-attention
 
     def create_and_check_with_lm_head(
         self,