* test=develop, fix docker with paddle nccl problem
* test=develop, refine en_doc for Variable and Program
* test=document_fix, fix English doc for Variable and Program
* test=document_fix, refine astype code block style
* test=document_fix, add example code for Variable properties
* test=document_fix, fix BackwardStrategy English Doc
* test=document_fix, fix syntax
* test=document_fix, refresh API.spec
* test=document_fix, refine api spec
* test=document_fix, refine api spec