platforms: Fix Ray DP startup crash (#36665)

Signed-off-by: Itay Alroy <ialroy@nvidia.com>

vllm/platforms/interface.py

@@ -638,6 +638,11 @@ class Platform:
         """Raises if this request is unsupported on this platform"""
 
     def __getattr__(self, key: str):
+        # Pickle checks dunder methods like __getstate__. If we return None
+        # for them, pickle treats it like a real value and tries to call it.
+        if key.startswith("__") and key.endswith("__"):
+            raise AttributeError(key)
+
         device = getattr(torch, self.device_type, None)
         if device is not None and hasattr(device, key):
             attr = getattr(device, key)