Quick Fix by adding conditional import for flash_attn_varlen_func in flash_attn (#20143)

Signed-off-by: Chendi.Xue <chendi.xue@intel.com>

vllm/attention/utils/fa_utils.py

@@ -66,3 +66,7 @@ def get_flash_attn_version(requires_alibi: bool = False) -> Optional[int]:
 def flash_attn_supports_fp8() -> bool:
     return get_flash_attn_version() == 3 and \
         current_platform.get_device_capability().major == 9
+
+
+def is_flash_attn_varlen_func_available() -> bool:
+    return current_platform.is_cuda() or current_platform.is_xpu()

vllm/v1/attention/backends/flash_attn.py

@@ -14,10 +14,14 @@ from vllm.attention.backends.abstract import (AttentionBackend, AttentionImpl,
 from vllm.attention.layer import Attention
 from vllm.attention.ops.merge_attn_states import merge_attn_states
 from vllm.attention.utils.fa_utils import (flash_attn_supports_fp8,
-                                           flash_attn_varlen_func,
                                            get_flash_attn_version,
-                                           get_scheduler_metadata,
-                                           reshape_and_cache_flash)
+                                           is_flash_attn_varlen_func_available)
+
+if is_flash_attn_varlen_func_available():
+    from vllm.attention.utils.fa_utils import (flash_attn_varlen_func,
+                                               get_scheduler_metadata,
+                                               reshape_and_cache_flash)
+
 from vllm.config import VllmConfig, get_layers_from_vllm_config
 from vllm.logger import init_logger
 from vllm.utils import cdiv