Fix cycle dependencies, Fix #3583.

* Also simplify pybind implementation by using OperatorBase as holder
  type.

* Clone method will create a new object instance, which is as same as
  itself.
* This is the first step to remove shared_ptr for OperatorBase

* insert rnn's backward into Backward()

* add device_context into backward_test

* make stepnet member

* add pybind support

* fix Inputs Outputs

* remove unique_ptr

* move namespace rnn to a directory

* Make interface of Operator to `const Scope&`

* the tensor only contains shape and does not hold memory when inferring shape.

* 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.