* branch init

* logical graph

* chain graph with bp struct

* backward done

* loss record struct

* build model struct

* compute task node

* fix bugf

* GenSortedCompTaskNodes

* clone op with boxing thread

* BuildSubTaskGraphMethod

* remove addclone for data_loader

* copy task node

* task graph build done

* fix boxing

* add TODO

* update

* update

* MaxModelSplitNum

* data part num

* DataConcatAndDataSplit


Former-commit-id: 8ed98d5c

Former-commit-id: f3f8d393

Former-commit-id: f60d2481

* remove tensorflow::io and tensorflow::str_util in project

* add str_util.h

* add Dirname and Str_util_test

* prepare cmake file for windows

* add 'third_party' directory

* make the third_party: gflags/glog/googletest/grpc/protobuf work

* fix cmake issues on linux

* refine library_dir and build_library_dir

* make the windows build pass

* add oneflow_external_dependencies

* fix add_external_dependencies

* fix the static lib linking order issue on Linux

* remove useless file in cmake directory

* add simple build steps in README.md

* add simple build steps in README.md

* fix markdown end of line

* add filters for visual studio project

* use sub directory to enumerate the filters in Visual Studio, add
'proto' to the oneflow project

* move CMakeLists.txt to the root dir and update README.md

* fix merge issue and change c++ standard to 11.