Disable the finding of python, swig related packages completely when WITH_PYTHON and WITH_SWIG_PY are set to OFF.

* insert rnn's backward into Backward()

* add device_context into backward_test

* large model train note

* update

* update doc

fix crash when disable WITH_SWIG_PY

build documentation don't need install Paddle before

Fix local recordio reader

add shared mkl lib in whl

* make stepnet member

* add pybind support

* fix Inputs Outputs

* remove unique_ptr

change operator public to protected

scatter update implemented

Replace `EmptyOp` with `NOP`

simplify docker script

Merge maps in OpRegistry and simplify register macros

Fix pnpair evaluator bug