Rewrite activation function (#5465)

* add activation

* rename swish to silu

* add selu

* add four activation op

* add softsign test

* add silu mish selu softsign

* Add softsign docs

* Add functional impl

* small fix for softsign backward

* remove flow.mish test

* add silu module test

* add selu test

* fix docs

* fix softsign docs

* fix format

* fix static cast

* merge master functional api yaml

* add torch style unittest

* Remove assert and add torch unittest

* add tensor def

* remove softsign test temporary

* add return maybe ok

* migrate nn ops to single_client dir

* migrate unittest

* remove lazy unittest

* add unittest

* fix to new directory

* Remove useless docs and single client test

* add doc

* fix docs

* add docs in oneflow and tensor namespace

* add torch autotest

* fix to new autotest

* remove outdated python code

* remove useless docs

* enlarge unittest tolerance

* Add static cast for const value

* skip softsign unittest

* skip tensor softsign

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