Pipeline Parallelism by stage buffer (#4666)

* Pipeline Parallelism: checkpointing insert identity buffer op

* fix complier err

* identity buffer op custom out regst num

* fix bug and runnable

* Chain merge divide fw/bw; MemChain ignore merge; copyhd regst num hack

* Pipeline buffer pass

* Pipeline runnable

* rollback NOT merge mem chain hack

* pipeline_stage_id_hint and rollback checkpointing buffer

* Pipeline buffer only. test pass.

* rollback repeat hack

* Remove CopyHd Hack; Add buffer cross label loader and loss

* refine code for review & fix for new dtype infer

* add note

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