* add rnn op interfaces
* add Run
* rename state -> memory
* change state -> memory
* make compilable
* add .cc
* init test
* add op fake implementation
* add CreateStepNet and CreateScopes implementation.
* add TODO list
* init memory attributes.
* add LinkMemories
* add PlainNet fake implementation
* Use std::shared_ptr<Scope> in the OpRunContext.
* add test
* disable mutable_data
* finist segmentInput function
* enable mutable_data with a trick
* RNNOp test.
* enable LinkMemories with mutable_data
* update SegmentInput function with comments
* finish ConcatOutput function
* reformat inputs and attributes
boot_memories
* Refine unit test.
* Refine unit test.
* modify inlinks.
* add OpDesc to Net
* fix bug and update unit test.
* move step scopes from inputs to outputs
* fix merge conflict, update SegmentInput function
* add RecurrentOpProtoAndCheckerMaker.
* clean the codes
* Abstract GetStepScopes and GetMaxSeqLen function
* refine LinkMemories
* Refine code and add some comments.
* add backward core
* update for develop branch.
* add forward core
* add forward algorithm
* Add RecurrentGradientAlgorithm implenmention.
* use CopyFrom and Slice function in RecurrentOp
* add unit test for LinkMemories.
* fix unit test.
* use the latest tensor.h, solve conflict
* add maker
* move SegmentInput and ConcatOutput to details nameplace
* unit test for RecurrentGradientAlgorithm.
* apply OperatorBase
* apply net operator.
* move memorys to attributes
* add RecurrentGradientOp
* open test unit test in recurrent_network_op_test.
* revert some files.
* add RecurrentArgument and Link struct to simplify member variable.
* rename.
* move recurrent_op from framework to operators
* add RecurrentGradientOp Init
* fix name
* fix Link.interal/external name
* use namespace operators instead of framework
* clean the code
* use the latest add_op and mul_op, don't test backward now
* Remove ScopePtr and OperatorPtr
* add get_net to pybind
* add test_recurrent_op.py
* add random into gen_tensor
* update to develop branch and refine some code.
* add some comments.