Created by: liu0haha123

反向传播过程错误

• 版本、环境信息：    1）PaddlePaddle版本：1.8.2    2）CPU：    3）GPU：P100 cuda10.1    4）系统环境：ubuntu16.04
• 训练信息    1）单机单卡    2）显存信息    3）Operator信息
• 复现信息：由于模型结构问题，loss的计算函数和forward函数的前向传播过程是分别写在两个不同的函数中的，训练时调用的是模型的自己编写的cal_loss函数而不是forward函数，是不是这种原因产生的 报错信息：

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C++ Call Stacks (More useful to developers):
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0   std::string paddle::platform::GetTraceBackString<std::string const&>(std::string const&, char const*, int)
1   paddle::platform::EnforceNotMet::EnforceNotMet(std::string const&, char const*, int)
2   paddle::imperative::DygraphInferShapeContext<paddle::imperative::VariableWrapper>::SetOutputDim(std::string const&, paddle::framework::DDim const&)
3   paddle::operators::LstmUnitGradOp::InferShape(paddle::framework::InferShapeContext*) const
4   paddle::imperative::PreparedOp::Run(std::map<std::string, paddle::imperative::SavedVariableWrapperList, std::less<std::string>, std::allocator<std::pair<std::string const, paddle::imperative::SavedVariableWrapperList> > > const&, std::map<std::string, paddle::imperative::SavedVariableWrapperList, std::less<std::string>, std::allocator<std::pair<std::string const, paddle::imperative::SavedVariableWrapperList> > > const&, std::unordered_map<std::string, boost::variant<boost::blank, int, float, std::string, std::vector<int, std::allocator<int> >, std::vector<float, std::allocator<float> >, std::vector<std::string, std::allocator<std::string> >, bool, std::vector<bool, std::allocator<bool> >, paddle::framework::BlockDesc*, long, std::vector<paddle::framework::BlockDesc*, std::allocator<paddle::framework::BlockDesc*> >, std::vector<long, std::allocator<long> >, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_>, std::hash<std::string>, std::equal_to<std::string>, std::allocator<std::pair<std::string const, boost::variant<boost::blank, int, float, std::string, std::vector<int, std::allocator<int> >, std::vector<float, std::allocator<float> >, std::vector<std::string, std::allocator<std::string> >, bool, std::vector<bool, std::allocator<bool> >, paddle::framework::BlockDesc*, long, std::vector<paddle::framework::BlockDesc*, std::allocator<paddle::framework::BlockDesc*> >, std::vector<long, std::allocator<long> >, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_> > > > const&)
5   paddle::imperative::OpBase::Run(paddle::framework::OperatorBase const&, std::map<std::string, paddle::imperative::SavedVariableWrapperList, std::less<std::string>, std::allocator<std::pair<std::string const, paddle::imperative::SavedVariableWrapperList> > > const&, std::map<std::string, paddle::imperative::SavedVariableWrapperList, std::less<std::string>, std::allocator<std::pair<std::string const, paddle::imperative::SavedVariableWrapperList> > > const&, std::unordered_map<std::string, boost::variant<boost::blank, int, float, std::string, std::vector<int, std::allocator<int> >, std::vector<float, std::allocator<float> >, std::vector<std::string, std::allocator<std::string> >, bool, std::vector<bool, std::allocator<bool> >, paddle::framework::BlockDesc*, long, std::vector<paddle::framework::BlockDesc*, std::allocator<paddle::framework::BlockDesc*> >, std::vector<long, std::allocator<long> >, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_>, std::hash<std::string>, std::equal_to<std::string>, std::allocator<std::pair<std::string const, boost::variant<boost::blank, int, float, std::string, std::vector<int, std::allocator<int> >, std::vector<float, std::allocator<float> >, std::vector<std::string, std::allocator<std::string> >, bool, std::vector<bool, std::allocator<bool> >, paddle::framework::BlockDesc*, long, std::vector<paddle::framework::BlockDesc*, std::allocator<paddle::framework::BlockDesc*> >, std::vector<long, std::allocator<long> >, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_> > > > const&, paddle::platform::Place const&)
6   paddle::imperative::BasicEngine::Execute()

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Error Message Summary:
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NotFoundError: can not find [C_prev@GRAD] in output
  [Hint: Expected it != var_base_map_out_->end(), but received it == var_base_map_out_->end().] at (/paddle/paddle/fluid/imperative/infer_shape_context.h:274)
  File "/root/IE_dygraph/train.py", line 69, in train
    loss.backward()
  File "/root/IE_dygraph/train.py", line 79, in <module>
    train(epochs=1)