* graph api

* add graph dummy test

* add test

* add recursive module mode

* graph.build test pass

* add detail check on graph inner node

* support config and train

* add repr for debug

* test buffer

* test buffer add

* refine test

* add comment

* refine test

* refactor Node to Block

* add named_state

* refine Graph.named_state()

* add state_tensortuple

* graph._compile()

* add mc session 0

* nn.graph: state tuple to private var; add BlockType; add simple multi client session

* NNGraphIf

* rm old graph.cpp

* nn.graph: add cpp NNGraph; export and call NNGraph

* add comment

* nn.Graph: rm prototype MultiClientSession

* nn.Graph: rm prototype MultiClientSession test

* nn.Graph: add TODO

* nn.Graph: format for review

* nn.Graph: format

* nn.Graph: format

* nn.Graph: pass flake8 check

Co-authored-by: Xinqi Li <lixinqi0703106@163.com>
Co-authored-by: oneflow-ci-bot <69100618+oneflow-ci-bot@users.noreply.github.com>
Co-authored-by: cheng cheng <472491134@qq.com>