[Refactor] Consolidate sequence normalization and enc-dec parsing (#33928)

Signed-off-by: DarkLight1337 <tlleungac@connect.ust.hk>

tests/entrypoints/openai/test_completion_error.py

@@ -53,6 +53,7 @@ class MockModelConfig:
     generation_config: str = "auto"
     media_io_kwargs: dict[str, dict[str, Any]] = field(default_factory=dict)
     skip_tokenizer_init = False
+    is_encoder_decoder: bool = False
 
     def get_diff_sampling_param(self):
         return self.diff_sampling_param or {}