[Tools] Support replacing the ${key} with the value of cfg.key

[Czm369]

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Motivation

Replace the ${key} with the value of ori_cfg.key in the config. And support replacing the chained ${key}. Such as, replace ${key0.key1} with the value of cfg.key0.key1.

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)

The origin config with "${key}":

ori_cfg_dict = dict()
ori_cfg_dict['work_dir'] = 'work_dirs/${percent}/${fold}'
ori_cfg_dict['percent'] = 5
ori_cfg_dict['fold'] = 1
ori_cfg_dict['model_wrapper'] = dict(type='SoftTeacher', detector='${model}')
ori_cfg_dict['model'] = dict(
        type='FasterRCNN',
        backbone=dict(type='ResNet'),
        neck=dict(type='FPN'),
        rpn_head=dict(type='RPNHead'),
        roi_head=dict(type='StandardRoIHead'),
        train_cfg=dict(
            rpn=dict(
                assigner=dict(type='MaxIoUAssigner'),
                sampler=dict(type='RandomSampler'),
            ),
            rpn_proposal=dict(nms=dict(type='nms', iou_threshold=0.7)),
            rcnn=dict(
                assigner=dict(type='MaxIoUAssigner'),
                sampler=dict(type='RandomSampler'),
            ),
        ),
        test_cfg=dict(
            rpn=dict(nms=dict(type='nms', iou_threshold=0.7)),
            rcnn=dict(nms=dict(type='nms', iou_threshold=0.5)),
        ),
    )
ori_cfg_dict['iou_threshold'] = dict(
        rpn_proposal_nms='${model.train_cfg.rpn_proposal.nms.iou_threshold}',
        test_rpn_nms='${model.test_cfg.rpn.nms.iou_threshold}',
        test_rcnn_nms='${model.test_cfg.rcnn.nms.iou_threshold}',
    )
ori_cfg_dict['a'] = 'xxxxx${b}xxxxx'
ori_cfg_dict['b'] = 'Hello, world!'
ori_cfg = Config(ori_cfg_dict, filename=cfg_path)

After replacing the ${key} with the corresponding value.

updated_cfg = replace_cfg_vals(deepcopy(ori_cfg))

The updated_cfg satisfies that

updated_cfg.work_dir ==  'work_dirs/5/1'
updated_cfg.model.detector == ori_cfg.model
updated_cfg.iou_threshold.rpn_proposal_nms == ori_cfg.model.train_cfg.rpn_proposal.nms.iou_threshold
updated_cfg.a  == 'xxxxxHello, world!xxxxx'

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.

[codecov]

Codecov Report

Merging #7492 (5c1a503) into dev (b1f40ef) will increase coverage by 0.01%.
The diff coverage is 96.77%.

@@            Coverage Diff             @@
##              dev    #7492      +/-   ##
==========================================
+ Coverage   64.89%   64.90%   +0.01%     
==========================================
  Files         351      352       +1     
  Lines       28466    28497      +31     
  Branches     4807     4819      +12     
==========================================
+ Hits        18472    18495      +23     
- Misses       9014     9024      +10     
+ Partials      980      978       -2     

Flag	Coverage Δ
unittests	64.87% <96.77%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmdet/utils/replace_cfg_vals.py	96.66% <96.66%> (ø)
mmdet/utils/__init__.py	100.00% <100.00%> (ø)
mmdet/models/detectors/cornernet.py	94.87% <0.00%> (-5.13%)	⬇️
mmdet/models/dense_heads/corner_head.py	68.33% <0.00%> (-1.39%)	⬇️

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[hhaAndroid]

LGTM