* export Backward to python

Fix #3171

Add get_numeric_gradient API and its unittest.

* Make interface of Operator to `const Scope&`

* 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.

* Refine unittest also

* ScopePtr means pointer of scope, but it can be shared or uniqued.
Change it to std::shared_ptr<Scope> to make code better to read.

* Expose Net to Python

* Expose PlainNet to Python, make python can add_op, complete_add_op
* Provide a low level api to manipulate Net
* Unittest for Net::DebugString

* add cross-entropy-op

* add infershape and compute

* implement Infershape and compute of onehotcrossentropy op

* a simplest SGD op

* Expose PlainNet to Python, make python can add_op, complete_add_op
* Provide a low level api to manipulate Net
* Unittest for Net::DebugString

ATTENTION: some interfaces changed:
1. void Tensor::set_dims(const DDim& dims) ==> void Tensor::Resize(const DDim& dims).
2. void Tensor::ShareDataFrom(const Tensor& src)  ==> void Tensor::ShareDataWith(const Tensor& src)
3. DDim Tensor::dims() const ==> const DDim& Tensor::dims() const