[diffusion] refactor: refactor attention backend checking to use backend enum (#15555)

Signed-off-by: Xiaodong Ye <yeahdongcn@gmail.com>

python/sglang/multimodal_gen/runtime/platforms/interface.py

@@ -23,6 +23,7 @@ logger = init_logger(__name__)
 
 
 class AttentionBackendEnum(enum.Enum):
+    FA2 = enum.auto()
     FA = enum.auto()
     SLIDING_TILE_ATTN = enum.auto()
     TORCH_SDPA = enum.auto()