[Bugfix] Fix sleep mode level 2 (#1376)

### What this PR does / why we need it?
For sleep mode level 2, we discarded model both weights and kv_cache,
but the problems is: When we discard weights, we also discard some
tensors representing the model state which we called
`model.named_buffers()`, such as: `running_mean / running_var` in
BatchNorm、rope cos-sin cache ... when we update weights, but forgot to
update buffers as well, this will lead to some unknown issue
### Does this PR introduce _any_ user-facing change?

### How was this patch tested?


- vLLM version: v0.10.2
- vLLM main:
5963b98b46

---------

Signed-off-by: wangli <wangli858794774@gmail.com>

tests/ut/worker/test_worker_v1.py

@@ -258,7 +258,7 @@ class TestNPUWorker(TestBase):
         # Create worker mock
         with patch.object(NPUWorker, "__init__", lambda x, **kwargs: None):
             worker = NPUWorker()
-
+            worker._sleep_saved_buffers = {}
             # Test wake_up method
             worker.wake_up(tags=["test_tag"])