[Enhance] Add init_cfg with type='pretrained' to downstream tasks

[fangyixiao18]

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Motivation

Related issue: (#1715)
Try to avoid the ACC drop due to the missed key 'prefix=backbone.'

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 repositories?
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

Before PR:

• Pre-commit or other linting tools are used to fix the potential lint issues.
• Bug fixes are fully covered by unit tests, the case that causes the bug should be added in the unit tests.
• The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, like docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with those projects, like MMDet or MMSeg.
• CLA has been signed and all committers have signed the CLA in this PR.

[codecov]

Codecov Report

Patch coverage: 48.49% and project coverage change: -2.78 ⚠️

Comparison is base (f9dcae2) 68.16% compared to head (292912d) 65.39%.

❗ Current head 292912d differs from pull request most recent head f191ca5. Consider uploading reports for the commit f191ca5 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1717      +/-   ##
==========================================
- Coverage   68.16%   65.39%   -2.78%     
==========================================
  Files         295      330      +35     
  Lines       23372    25744    +2372     
  Branches     3713     4111     +398     
==========================================
+ Hits        15932    16835     +903     
- Misses       6880     8290    +1410     
- Partials      560      619      +59     

Flag	Coverage Δ
unittests	65.39% <48.49%> (-2.78%)	⬇️

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

Impacted Files	Coverage Δ
mmpretrain/apis/feature_extractor.py	37.50% <0.00%> (ø)
mmpretrain/apis/image_caption.py	30.64% <0.00%> (ø)
mmpretrain/apis/image_retrieval.py	21.42% <0.00%> (ø)
mmpretrain/apis/visual_grounding.py	27.53% <0.00%> (ø)
mmpretrain/apis/visual_question_answering.py	25.67% <0.00%> (ø)
mmpretrain/datasets/__init__.py	60.46% <0.00%> (-13.83%)	⬇️
mmpretrain/datasets/flickr30k_caption.py	0.00% <0.00%> (ø)
mmpretrain/datasets/flickr30k_retrieval.py	0.00% <0.00%> (ø)
mmpretrain/datasets/gqa_dataset.py	0.00% <0.00%> (ø)
mmpretrain/datasets/nocaps.py	0.00% <0.00%> (ø)
... and 38 more

... and 30 files with indirect coverage changes

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