neural machine translation model support beam search with while op

* limit variable type to lod tensor in memory optimization transpiler

* refine policy

* support while operator

* fix random seed and training data order

* refine get_cfgs method to support multi while operators

* refine codes

* refine conv2d_transpose layer doc

* fix conv2d_transpose doc

* fix doc

* Updating the cluster trainign doc

* Fixed few comments of transpiler

* Adding few explanations

2. Fix evaluator using 'reduce_sum' op instead of 'mean' op

* also div num_samples when return cost of nce_op

* add memory optimization transpiler demo

* add memory benchmark compile option

* add gflags instead of macro

* refine code

And write an unittest for it

* "fix decode bug"

* "follow commnet"

* "fix error"

* "fix hook bug"

* fix based comment

* fix copyright

* fix based on comment