[Bugfix] Fix try-catch conditions to import correct Flash Attention Backend in Draft Model (vllm-project#9101)

Signed-off-by: LeiWang1999 <leiwang1999@outlook.com>

Author: tjtanaa

vllm/spec_decode/draft_model_runner.py

@@ -6,11 +6,16 @@
 from vllm.model_executor.layers.sampler import SamplerOutput
 
 try:
-    from vllm.attention.backends.flash_attn import FlashAttentionMetadata
-except ModuleNotFoundError:
-    # vllm_flash_attn is not installed, use the identical ROCm FA metadata
-    from vllm.attention.backends.rocm_flash_attn import (
-        ROCmFlashAttentionMetadata as FlashAttentionMetadata)
+    try:
+        from vllm.attention.backends.flash_attn import FlashAttentionMetadata
+    except (ModuleNotFoundError, ImportError):
+        # vllm_flash_attn is not installed, try the ROCm FA metadata
+        from vllm.attention.backends.rocm_flash_attn import (
+            ROCmFlashAttentionMetadata as FlashAttentionMetadata)
+except (ModuleNotFoundError, ImportError) as err:
+    raise RuntimeError(
+        "Draft model speculative decoding currently only supports"
+        "CUDA and ROCm flash attention backend.") from err
 
 from vllm.config import (CacheConfig, DeviceConfig, LoadConfig, LoRAConfig,
                          ModelConfig, ObservabilityConfig, ParallelConfig,