Fix cycle dependencies, Fix #3583.

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

Fit google C++ style

Make code more clearer

* 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

* remove SliceCopied

* remove SliceCopied

* rename SliceShared to SliceLevels, SliceInLevel

* merge lod_tensor/details

* remove lod_start_pos_'s shared_ptr

* make lod-tensor a special tensor

* add clone to lod_tensor

* add lod tensor impl

* add lodtensor clone test

* init lod

* add equal

* merge LOD and its methods

* recover tensor and variable

* change thrust to host_vector