* refactor(Optim): refine optimizer codes

* docs(SGD): add document for SGD

* docs(SGD): fix code

* test(Adam): fix test_optim_adam bug

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

* feat(Optim): add sgd

* fix(*): add no_grad guard in optimizer

* fix(*): fix bug in optimizer test

* style(*): format codes

* style(Optimizer): refine codes

* style(Optimizer): refine codes

* fix(Optimizer): fix bug in ParamGroup

* fix(*): add __init__.py

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