Created by: Aurelius84

As the title.

Example code:

Class SubNet(fluid.dygraph.Layer):

    @dygraph_to_static_graph
    def forward(self, x)
        return self.fc(x)

Class SubNet(fluid.dygraph.Layer):
    def __init__(self):
        self.fc_net = SubNet()

    @dygraph_to_static_graph
    def forward(self, x)
        # Convert into static_func at first time
        fc_1 = self.fc_net(x)
        
        # Convert into static_func repeatly before this PR.
        # In this PR, here cached static_func will be used.
        fc_2 = self.fc_net(x)