Add inner register backward hook method for Tensor

[chenwhql]

PR types

New features

PR changes

APIs

Describe

Add inner register backward hook method for Tensor

• remove InplaceVariableWrapperHook class and related method
• add simple void Hook for reducing in distributed training
• add Tensor._register_backward_hook method in python
  • used for reducer like in c++ for distributed training

var->GradVarBase()->AddVoidHook(std::make_shared<std::function<void()>>(
        [=]() { this->AddDistHook(global_var_index); }));

• example

HOOK_INIT_VALUE = 10

def global_void_hook():
    global HOOK_INIT_VALUE
    HOOK_INIT_VALUE *= 2

x = paddle.to_tensor(5., stop_gradient=False)
x._register_backward_hook(global_void_hook)

for i in range(5):
    y = paddle.pow(x, 4.0)
    y.backward()

[paddle-bot-old]

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[ForFishes]

LGTM