* use yapf to format all python file

* yapf exclude two unittests file for they rely on writing and reading file, and format will break them

* disable diff_py_file because too many diff files cause command following failed

* improve introduction of bfgs args; test=document_fix

* modify according to zhouwei's comment; test=document_fix

* fix bfgs_doc; test=document_fix

* add parameter name; test=document_fix

* modify according to chenlong's comments;test=document_fix

* cut off relation between xk and initial_position's graph

* fix_bug

* add detach to cut off with original graph

* [New API]: miminize_bfgs and miminize_lbfgs

* modify for python module call correctly

* add functional package, add error raise in static_graph, change assign to set_value

* unify static_graph and dygraph, fix bug when x or H0 is float64

* now only accept input is tensor, put check args in utils.py, put exception test together

* temp

* add more detailed algorithm illustration and comment, reduce test case to limit test time in 15s

* change in_dygraph_mode to in_dynamic_mode

* fix bug of sample code; reduce test case to reduce test time

* change dir to incubate