Created by: guoshengCS

在append_backward_vars的时候rnn_memory_helper_grad引入的grad var可能会被漏掉。 修改并增加单测，未修改前该单测报如下错误：

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Error Message Summary:
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Error: Cannot find in_grad_var in scope, name is dropout_1.tmp_0@GRAD at (/Users/guosheng/Documents/codeRep/paddle-pr-new/Paddle/paddle/fluid/operators/rnn_memory_helper_op.cc:97)
  [operator < rnn_memory_helper_grad > error]