[Bugfix] Disable cross-layer KV cache for MLA attention backends (#37090)

Signed-off-by: haosdent <haosdent@gmail.com>
Co-authored-by: Or Ozeri <oro@il.ibm.com>

vllm/v1/attention/backends/mla/indexer.py

@@ -63,6 +63,9 @@ class DeepseekV32IndexerBackend(AttentionBackend):
         include_num_layers_dimension: bool = False,
     ) -> tuple[int, ...]:
         if include_num_layers_dimension:
+            # DeepseekV32Indexer kernels do not support cross-layer
+            # KV cache layout. Identity permutation keeps num_layers
+            # first, signaling incompatibility.
             return (0, 1, 2, 3)
         return (0, 1, 2)