[Enhance] Sthv2 checkpoint for TSN/TSM

[hukkai]

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Motivation

Please describe the motivation of this PR and the goal you want to achieve through this PR.

Modification

Please briefly describe what modification is made in this PR.

BC-breaking (Optional)

Does the modification introduces changes that break the back-compatibility of this repo?
If so, please describe how it breaks the compatibility and how users should modify their codes 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 should be used to fix the potential lint issues.
2. The modification should be covered by complete unit tests. If not, please add more unit tests 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 should be modified accordingly, like docstring or example tutorials.

[Dai-Wenxun]

Update README.md to align with VIdeoMAE

[codecov]

Codecov Report

Base: 76.00% // Head: 76.65% // Increases project coverage by +0.65% 🎉

Coverage data is based on head (ef06ef8) compared to base (a0863e9).
Patch has no changes to coverable lines.

Additional details and impacted files

@@             Coverage Diff             @@
##           dev-1.x    #2082      +/-   ##
===========================================
+ Coverage    76.00%   76.65%   +0.65%     
===========================================
  Files          137      139       +2     
  Lines         9918     9964      +46     
  Branches      1626     1631       +5     
===========================================
+ Hits          7538     7638     +100     
+ Misses        2029     1972      -57     
- Partials       351      354       +3     

Flag	Coverage Δ
unittests	76.65% <ø> (+0.65%)	⬆️

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

Impacted Files	Coverage Δ
mmaction/models/backbones/swin.py	93.06% <0.00%> (-0.22%)	⬇️
mmaction/testing/_utils.py	45.56% <0.00%> (ø)
mmaction/testing/__init__.py	100.00% <0.00%> (ø)
mmaction/models/heads/base.py	80.00% <0.00%> (+2.35%)	⬆️
mmaction/models/recognizers/base.py	64.55% <0.00%> (+3.84%)	⬆️
mmaction/models/backbones/resnet2plus1d.py	100.00% <0.00%> (+4.00%)	⬆️
mmaction/models/backbones/vit_mae.py	95.45% <0.00%> (+4.75%)	⬆️
mmaction/models/recognizers/recognizer2d.py	59.61% <0.00%> (+9.61%)	⬆️
mmaction/utils/gradcam_utils.py	97.72% <0.00%> (+88.53%)	⬆️

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