[Enhance] Call register_all_modules in Registry.get()

[HAOCHENYE]

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Motivation

If we use config like this:

model = dict(type='RetinaNet'
    backbone=dict(type='mmcls.ResNet'
    ...
    )
)

MODELS.build(model) will NOT regist modules of mmcl, and we need to set custom_configs=... in config file extraly.

However, if we set _scope_ in config:

model = dict(type='RetinaNet'
    _scope_='mmcls',
    backbone=dict(type='ResNet'
    ...
    )
)

The modules of mmcls will be registered, and its behavior is inconsistent with type=mmcls.ResNet.
Therefore, this pr make type=mmcls.ResNet will also trigger the regist process.

Modification

Please briefly describe what modification is made in this PR.

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repos?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here, and update the documentation.

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMCls.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[RangiLyu]

Please fix the typos