Created by: JepsonWong

This PR support backward hook for dygraph.

The backward hook will be called every time a gradient will respect to the Variable is computed. The hook shold have the following signature:

• hook(grad) -> Variable or None The hook should not modify its param, but it can optionally return a new gradient which will change the Variable's value.

register_hook function will return a RemovablePyCallableObject object with method remove that can remove the hook from the VarBase.

execute_hooks function will exectute the hook functions from the Variable.

import paddle.fluid as fluid
import numpy as np

def hook1(g):
        g = 2 * g
        print g
        return g

def hook2(g):
        g = 4 * g
        print g

with fluid.dygraph.guard():
        # var's type is VarBase
        remove_hook1 = var.register_hook(hook1)
        remove_hook2 = var.register_hook(hook2)
        remove_hook1.remove()
        remove_hook2.remove()