* add flow.randn

* auto format by CI

* refine

* add mean and std as attr to normal_op/kernel/distribution

* refine

* refine

* keep module, fix docstring to pass CI

* refine
Co-authored-by: Noneflow-ci-bot <ci-bot@oneflow.org>
Co-authored-by: NYao Chi <later@usopp.net>
Co-authored-by: Noneflow-ci-bot <69100618+oneflow-ci-bot@users.noreply.github.com>