Add flatten module and unit test (#4759)

* add flatten module and unit test

* add changes according to review

* fix docs

* use nn.init

* add modification according to review

* remove useless code

* add more tests & fix doc

* fix default parameters for module and function

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