[CI] add trainer_send_weights for MockWeightTransferEngine (#35589)

Signed-off-by: chaunceyjiang <chaunceyjiang@gmail.com>

tests/entrypoints/weight_transfer/test_weight_transfer_llm.py

@@ -90,6 +90,10 @@ class MockWeightTransferEngine(WeightTransferEngine[MockInitInfo, MockUpdateInfo
     def shutdown(self) -> None:
         MockWeightTransferEngine.shutdown_called = True
 
+    def trainer_send_weights(self, *args, **kwargs):
+        """Mock method to simulate trainer sending weights."""
+        pass
+
 
 def mock_create_engine(config, parallel_config):
     """Mock factory function that returns our mock engine."""