[CI] Fix: pass string cache_dtype in test_register_kv_caches (#38836)

tests/v1/kv_connector/unit/test_nixl_connector.py

@@ -1613,7 +1613,7 @@ def test_register_kv_caches(
                                 )
                             ]
                         ],
-                        cache_dtype=torch.bfloat16,
+                        cache_dtype="bfloat16",
                         device=torch.accelerator.current_device_index(),
                         kernel_block_sizes=[block_size],
                     )