Created by: songyouwei

新增enable_dygraph和disable_dygraph接口，支持函数式启动关闭动态图模式

sample code:

import paddle.fluid as fluid
import numpy as np

data = np.random.uniform( -1, 1, [30, 10, 32] ).astype('float32')

fluid.enable_dygraph()  # Now we are in dygragh mode
data = fluid.dygraph.to_variable(data, name='data')
print(fluid.in_dygraph_mode())  # True

fluid.disable_dygraph()
data = fluid.dygraph.to_variable(data, name='data')  # AssertionError