Fix gems DeviceDetector api (#236)

### PR Category
Core
### PR Type
Bug Fixes
### Description
Fix gems DeviceDetector api

Author: cyber-pioneer

vllm_fl/utils.py

@@ -5,7 +5,12 @@
 from typing import Optional, Tuple
 
 import flag_gems
-from flag_gems.runtime.backend.device import DeviceDetector
+try:
+    # FlagGems<=5.0.2: DeviceDetector lives in device.
+    from flag_gems.runtime.backend.device import DeviceDetector
+except ImportError:
+    # FlagGems>5.0.2: DeviceDetector lives in device_finder.
+    from flag_gems.runtime.backend.device_finder import DeviceDetector
 from flag_gems.runtime import backend
 
 _OP_CONFIG: Optional[dict[str, str]] = None