[HybridCache] Fix `get_seq_length` method (#31661)

* fix gemma2

* handle in generate

src/transformers/cache_utils.py

@@ -1083,7 +1083,7 @@ class HybridCache(Cache):
         # no matter how long the sentence is
         return self.max_cache_len
 
-    def get_seq_length(self, layer_idx: Optional[int] = 0) -> int:
+    def get_seq_length(self, layer_idx: Optional[int] = 0):
         return None
 
     def reset(self):

src/transformers/generation/utils.py

@@ -1399,7 +1399,7 @@ class GenerationMixin:
             cache = model_kwargs["past_key_values"]
             if not isinstance(cache, Cache):
                 past_length = cache[0][0].shape[2]
-            elif hasattr(cache, "get_seq_length"):
+            elif hasattr(cache, "get_seq_length") and cache.get_seq_length() is not None:
                 past_length = cache.get_seq_length()
 
         if "inputs_embeds" in model_kwargs: