flow.S/B/P (#5306)

* flow.S/B/P

* optimize

* optimize

* fix according comment

* add attr in experimental namespace

* optimize

* oneflow_export_value

* refine

* refine

* refine

* customized_symbol module

* refine

* fix bug

* Add new placement init func (#5408)

* add_new_placement_init_func

* flow.env.all_device_placement

* refine

* refine

* refine

* refine

* refine

* refine

* refine

* refine

* refine

Co-authored-by: oneflow-ci-bot <69100618+oneflow-ci-bot@users.noreply.github.com>