fix 2 bug of eager

[wanghuancoder]

PR types

Bug fixes

PR changes

Others

Description

Pcard-64703
修复两个动态图bug：

1. PyLayer在backward返回None后，grad_out添加了弄的Vector。在backward.cc中，如果Vector是空的会continue，导致None失效。修改方法为在Vector中加一个空的Tensor。
2. backward.cc，如果反向初始多个Tensor为同一个Node的输入时，Node会被执行多次。需要去重。

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[JiabinYang]

LGTM