[BUGFIX][Mamba][Qwen3.5] Zero freed SSM cache blocks on GPU (#35219)

Signed-off-by: Vadim Gimpelson <vadim.gimpelson@gmail.com>

vllm/v1/attention/backend.py

@@ -86,6 +86,26 @@ class AttentionBackend(ABC):
     ) -> tuple[int, ...]:
         raise NotImplementedError
 
+    @classmethod
+    def get_kv_cache_block_dim(
+        cls,
+        block_size: int,
+        num_kv_heads: int,
+        head_size: int,
+        cache_dtype_str: str = "auto",
+    ) -> int:
+        """Discover which tensor dim is the block index, since different
+        backends lay out dims differently."""
+        _S = 1234567
+        shape = cls.get_kv_cache_shape(
+            _S,
+            block_size,
+            num_kv_heads,
+            head_size,
+            cache_dtype_str=cache_dtype_str,
+        )
+        return shape.index(_S)
+
     @staticmethod
     def get_kv_cache_stride_order(
         include_num_layers_dimension: bool = False,