Add missing flag for CacheLayer (#39678)

* fix

* Update cache_utils.py

Author: Cyrilvallez

src/transformers/cache_utils.py

@@ -73,6 +73,8 @@ class DynamicLayer(CacheLayerMixin):
     See `CacheLayerMixin` for details on common methods that are implemented by all cache layers.
     """
 
+    is_sliding = False
+
     def update(
         self,
         key_states: torch.Tensor,
@@ -307,6 +309,8 @@ class SlidingWindowLayer(StaticLayer):
     See `CacheLayerMixin` for details on common methods that are implemented by all cache layers.
     """
 
+    is_sliding = True
+
     def __init__(self, sliding_window, *args, **kwargs):
         """
         Args: