[ROCm] Fix broken import in platform attention backend dispatching (#30432)

Signed-off-by: Andreas Karatzas <akaratza@amd.com>

vllm/platforms/rocm.py

@@ -403,7 +403,21 @@ class RocmPlatform(Platform):
                 compilation_config.cudagraph_mode = CUDAGraphMode.PIECEWISE
 
         if cache_config and cache_config.block_size is None:
-            cache_config.block_size = 16
+            if (
+                envs.VLLM_ROCM_USE_AITER_UNIFIED_ATTENTION and envs.VLLM_ROCM_USE_AITER
+                # NOTE: This block has been deprecated
+                # or get_env_variable_attn_backend()
+                # == AttentionBackendEnum.ROCM_AITER_UNIFIED_ATTN
+                # TODO: monitor https://github.com/vllm-project/vllm/pull/30396
+                # to see how we can transition to the new way of selecting
+                # attention backends
+            ):
+                cache_config.block_size = 64
+                logger.warning(
+                    "[ROCM_AITER_UNIFIED_ATTN]: Setting kv cache block size to 64."
+                )
+            else:
+                cache_config.block_size = 16
 
         if parallel_config.worker_cls == "auto":
             parallel_config.worker_cls = "vllm.v1.worker.gpu_worker.Worker"