[ROCm][Bugfix] fix cache block size mismatch for aiter unified attention (#37606)

Signed-off-by: Divakar Verma <divakar.verma@amd.com>

vllm/v1/attention/backends/rocm_aiter_unified_attn.py

@@ -29,6 +29,13 @@ class RocmAiterUnifiedAttentionBackend(RocmAttentionBackend):
     def get_supported_kernel_block_sizes() -> list[int | MultipleOf]:
         return [MultipleOf(16)]
 
+    @classmethod
+    def get_preferred_block_size(cls, default_block_size: int) -> int:
+        logger.warning_once(
+            "[ROCM_AITER_UNIFIED_ATTN]: Setting kv cache block size to 64."
+        )
+        return 64
+
     @classmethod
     def supports_block_size(cls, block_size: int | None) -> bool:
         if block_size is None: