• 版本、环境信息：
     1）PaddlePaddle版本：1.6.1
     2）CPU：Intel(R) Core(TM) i5-5257U CPU @ 2.70GHz
     4）系统环境：Mac OS 10.14.6，Python 3.6.9
• 训练信息
     1）单机
• 复现信息：如为报错，请给出复现环境、复现步骤

alpha = layers.create_parameter(shape=[1], dtype='float32', name='alpha', 
                                default_initializer=fluid.initializer.ConstantInitializer(1.))
alpha = alpha()

test_program = fluid.Program()
with fluid.program_guard(test_program):
    b = alpha * 2.0

place = fluid.CPUPlace()
fluid_executor = fluid.Executor(place)
fluid_executor.run(fluid.default_startup_program())

result = fluid_executor.run(test_program, feed={}, fetch_list=[b])[0]
print(result)

• 问题描述：报错信息

----------------------
Error Message Summary:
----------------------
PaddleCheckError: Cannot find fetch variable in scope, fetch_var_name is tmp_0 at [/home/teamcity/work/ef54dc8a5b211854/paddle/fluid/operators/controlflow/fetch_op.cc:38]
  [operator < fetch > error]