[Model Loading] Overlap checkpoint staging with CUDA graph capture during startup (#32017)

Co-authored-by: Wenhui Zhu <wzhu59@asu.edu>
Co-authored-by: Alex Nails <alex.nails@radixark.ai>

test/registered/unit/test_server_args_migration.py

@@ -93,6 +93,25 @@ class TestServerArgsAnnotatedCli(CustomTestCase):
                 sa = self._parse(["--image-processor-backend", backend])
                 self.assertEqual(sa.image_processor_backend, backend)
 
+    def test_startup_weight_load_mode(self):
+        """The startup loading mode keeps serial as the safe default."""
+        serial = self._parse([])
+        overlap = self._parse(["--startup-weight-load-mode", "overlap"])
+        self.assertEqual(serial.startup_weight_load_mode, "serial")
+        self.assertFalse(serial.is_startup_weight_load_overlap)
+        self.assertEqual(overlap.startup_weight_load_mode, "overlap")
+        self.assertTrue(overlap.is_startup_weight_load_overlap)
+
+        with self.assertRaises(SystemExit):
+            self.parser.parse_args(
+                [
+                    "--model",
+                    "dummy",
+                    "--startup-weight-load-mode",
+                    "unsupported",
+                ]
+            )
+
     def test_deprecated_flags_still_work(self):
         """Deprecated flags set the correct dest field."""
         sa = self._parse(["--stream-output"])