refactor(rocm): 提取 unified flash 的 block_size 判定逻辑

vllm/platforms/rocm.py

@@ -305,17 +305,17 @@ class RocmPlatform(Platform):
                 f"is not MLA type while requested for MLA backend."
             )
             
-        use_unified_flash = (
-            block_size is not None
-            and block_size != 64
+        is_non64_block_multiple_64 = (
+            block_size != 64
             and block_size % 64 == 0
+        )
+        use_unified_flash = (
+            is_non64_block_multiple_64
             and head_size == 256
         )
         if (
             envs.VLLM_USE_FLASH_ATTN_PA
-            and block_size is not None
-            and block_size != 64
-            and block_size % 64 == 0
+            and is_non64_block_multiple_64
             and head_size != 256
         ):
             logger.info_once(