GitLab

* [Enhance] Support register function.

* fix unittest error

* add docs and unittest of register function

* modify the docs

* fix version to 1.5.1

* Update docs/zh_cn/understand_mmcv/registry.md
Co-authored-by: Zaida Zhou <58739961+zhouzaida@users.noreply.github.com>

* Update docs/en/understand_mmcv/registry.md
Co-authored-by: Zaida Zhou <58739961+zhouzaida@users.noreply.github.com>

* refine the docs

* modify module_class to module
Co-authored-by: Zaida Zhou <58739961+zhouzaida@users.noreply.github.com>