[Attention][AMD] Make flash-attn optional (#30361)

Signed-off-by: Matthias Gehre <matthias.gehre@amd.com>

vllm/v1/attention/backends/fa_utils.py

@@ -23,11 +23,13 @@ elif current_platform.is_xpu():
 elif current_platform.is_rocm():
     try:
         from flash_attn import flash_attn_varlen_func  # noqa: F401
-    except ImportError as e:
-        raise ImportError(
-            "Rocm platform requires upstream flash-attn "
-            "to be installed. Please install flash-attn first."
-        ) from e
+    except ImportError:
+
+        def flash_attn_varlen_func(*args, **kwargs):
+            raise ImportError(
+                "ROCm platform requires upstream flash-attn "
+                "to be installed. Please install flash-attn first."
+            )
 
 
 def get_flash_attn_version(requires_alibi: bool = False) -> int | None: