[Refactor] Extract has_flash_attn_pkg in OmniPlatform (#3068)

Signed-off-by: Xiaodong Ye <yeahdongcn@gmail.com>
Co-authored-by: Canlin Guo <canlinguosdu@gmail.com>

vllm_omni/platforms/interface.py

@@ -105,6 +105,11 @@ class OmniPlatform(Platform):
         """Check if the platform supports torch.compile with inductor backend."""
         raise NotImplementedError
 
+    @classmethod
+    def has_flash_attn_package(cls) -> bool:
+        """Check if a Flash Attention package is available and usable on this platform."""
+        return False
+
     @classmethod
     def get_torch_device(cls, local_rank: int | None = None) -> torch.device:
         raise NotImplementedError