[Tools] Support respliting data_batch with tag

[Czm369]

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Motivation

Support respliting data_batch with tag.

Modification

Add a function in mmdet/utils/data_batch.py which can respliting data_batch with tag, such as 'sup', 'unsup_teacher', and 'unsup_student'.

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.

[codecov]

Codecov Report

Merging #7641 (be2ab02) into dev (c8270c3) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev    #7641      +/-   ##
==========================================
+ Coverage   64.92%   64.94%   +0.02%     
==========================================
  Files         348      349       +1     
  Lines       28323    28338      +15     
  Branches     4778     4783       +5     
==========================================
+ Hits        18389    18405      +16     
  Misses       8957     8957              
+ Partials      977      976       -1     

Flag	Coverage Δ
unittests	64.92% <100.00%> (+0.02%)	⬆️

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

Impacted Files	Coverage Δ
mmdet/utils/__init__.py	100.00% <100.00%> (ø)
mmdet/utils/split_batch.py	100.00% <100.00%> (ø)
mmdet/core/bbox/assigners/max_iou_assigner.py	73.68% <0.00%> (+1.31%)	⬆️

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[zytx121]

LGTM

[ZwwWayne]

Should resolve conflict

[BIGWangYuDong]

LGTM