fix gemma2

src/transformers/cache_utils.py

@@ -1084,7 +1084,11 @@ def get_max_length(self) -> Optional[int]:
         return self.max_cache_len
 
     def get_seq_length(self, layer_idx: Optional[int] = 0) -> int:
-        return None
+        """Returns the sequence length of the cached states that were seen by the model."""
+        # Occupied cache == any slot in the 3rd dim (sequence length) holds a non-zero value. To save on compute, let's
+        # limit the check to the first batch member and head dimension.
+        # TODO: deprecate this function in favor of `cache_position`
+        return (self.key_cache[layer_idx][0, 0].any(dim=-1)).sum()
 
     def reset(self):
         """Resets the cache values while preserving the objects"""