* add ZeroOptimizer to optim

* resolve `duplicate label` warnings

* upgrade docutils && shpinx to resolve `unknown directive or role` warnings

* fix typo

* resolve literal_block && heading warnings

* resolve json literal_block warnings

* resolve python literal_block warnings

* resolve bunches of reference warnings

* resolve bunches of docstring warnings

* resolve warnings in autosummary

* resolve remaining warnings in en docs

* resolve heading warnings in zh_cn docs

* resolve remaining warnings in zh_cn docs

* fix as comments

* fix as comments