[Enhance] Add 'get_ann_info' to dataset_wrappers.

[zhaoxin111]

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Motivation

add get_ann_info in RepeatDataset and ConcatDataset to support MultiImageMixDataset

Modification

add get_ann_info in RepeatDataset and ConcatDataset

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.

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All committers have signed the CLA.

[RangiLyu]

Hi, thanks for your contribution. We fixed python3.9 CI, please merge the latest master.

[RangiLyu]

Would you like to add get_ann_info to all other dataset wrapper so all of these wrappers can support MultiImageMixDataset?

[zhaoxin111]

Would you like to add get_ann_info to all other dataset wrapper so all of these wrappers can support MultiImageMixDataset?

As far as I know, only ClassBalancedDataset is not supported. I will try it.

[codecov]

Codecov Report

Merging #6352 (6872686) into master (db256a1) will increase coverage by 0.00%.
The diff coverage is 20.00%.

@@           Coverage Diff           @@
##           master    #6352   +/-   ##
=======================================
  Coverage   62.08%   62.09%           
=======================================
  Files         319      319           
  Lines       25304    25318   +14     
  Branches     4188     4191    +3     
=======================================
+ Hits        15711    15720    +9     
- Misses       8769     8770    +1     
- Partials      824      828    +4     

Flag	Coverage Δ
unittests	62.06% <20.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
mmdet/datasets/dataset_wrappers.py	76.21% <20.00%> (-4.97%)	⬇️
mmdet/core/bbox/assigners/max_iou_assigner.py	72.36% <0.00%> (-1.32%)	⬇️
mmdet/apis/inference.py	38.53% <0.00%> (+0.35%)	⬆️
mmdet/models/dense_heads/corner_head.py	69.46% <0.00%> (+1.40%)	⬆️
mmdet/models/detectors/cornernet.py	100.00% <0.00%> (+5.12%)	⬆️

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[RangiLyu]

One more thing: Would you like to add some unit tests?
Try to use MagicMock to mock the data_infos in the dataset. You can refer to the unit test of MultiImageMixDataset in tests/test_data/test_datasets/test_dataset_wrapper.py

[hhaAndroid]

Need to add a note explaining why it is needed.