fix dataset conflict issue in single machine simulation

paddle_fl/examples/femnist_demo/fl_master.py

@@ -25,7 +25,7 @@ model = Model()
 model.cnn()
 
 job_generator = JobGenerator()
-optimizer = fluid.optimizer.SGD(learning_rate=0.1)
+optimizer = fluid.optimizer.Adam(learning_rate=0.1)
 job_generator.set_optimizer(optimizer)
 job_generator.set_losses([model.loss])
 job_generator.set_startup_program(model.startup_program)