[sleep mode] clear pytorch cache after sleep (#15248)

Signed-off-by: <villard@us.ibm.com>

vllm/device_allocator/cumem.py

@@ -8,6 +8,7 @@
 # not sure why, they are created from a different context.
 # the only successful approach is to call cuda driver API in C.
 import dataclasses
+import gc
 import os
 from contextlib import contextmanager
 from typing import Any, Callable, Dict, Optional, Tuple, Union
@@ -204,6 +205,9 @@ class CuMemAllocator:
                 data.cpu_backup_tensor = cpu_backup_tensor
             unmap_and_release(handle)
 
+        gc.collect()
+        torch.cuda.empty_cache()
+
     def wake_up(self):
         """
         Wake up the allocator from sleep mode.