[Feature] Support return_hidden_states="last" (#30177)

Co-authored-by: litao.dream <litao.dream@bytedance.com>

test/registered/unit/server_args/test_server_args.py

@@ -42,6 +42,45 @@ _mock_device.start()
 
 
 class TestPrepareServerArgs(CustomTestCase):
+    def test_return_hidden_states_mode_configuration(self):
+        disabled = ServerArgs(model_path="dummy")
+        self.assertFalse(disabled.enable_return_hidden_states)
+        self.assertIsNone(disabled.return_hidden_states_mode)
+
+        last = ServerArgs(
+            model_path="dummy",
+            return_hidden_states_mode="last",
+        )
+        self.assertTrue(last.enable_return_hidden_states)
+        self.assertEqual(last.return_hidden_states_mode, "last")
+
+        legacy_full = ServerArgs(
+            model_path="dummy",
+            enable_return_hidden_states=True,
+        )
+        self.assertTrue(legacy_full.enable_return_hidden_states)
+        self.assertEqual(legacy_full.return_hidden_states_mode, "full")
+
+        parsed_last = prepare_server_args(
+            [
+                "--model-path",
+                "dummy",
+                "--return-hidden-states-mode",
+                "last",
+            ]
+        )
+        self.assertTrue(parsed_last.enable_return_hidden_states)
+        self.assertEqual(parsed_last.return_hidden_states_mode, "last")
+
+        with self.assertRaisesRegex(
+            ValueError,
+            "return_hidden_states_mode must be one of",
+        ):
+            ServerArgs(
+                model_path="dummy",
+                return_hidden_states_mode="lst",
+            )
+
     def test_config_nested_dict_args_are_json(self):
         with tempfile.NamedTemporaryFile(mode="w", suffix=".yaml", delete=False) as f:
             f.write("mm-process-config:\n  image:\n    resize: 128\n")