[Feature] Support PETR

[Ben-Louis]

Motivation

Modification

BC-breaking (Optional)

Use cases (Optional)

Checklist

Before PR:

• I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
• New functionalities are covered by complete unit tests. If not, please add more unit tests to ensure correctness.
• The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

• CLA has been signed and all committers have signed the CLA in this PR.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (2ab56bc) 81.17% compared to head (f3699ce) 81.16%.
Report is 47 commits behind head on dev-1.x.

❗ Current head f3699ce differs from pull request most recent head ce02f2c. Consider uploading reports for the commit ce02f2c to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           dev-1.x    #2418      +/-   ##
===========================================
- Coverage    81.17%   81.16%   -0.02%     
===========================================
  Files          226      226              
  Lines        13963    13963              
  Branches      2415     2415              
===========================================
- Hits         11335    11333       -2     
- Misses        2013     2015       +2     
  Partials       615      615              

Flag	Coverage Δ
unittests	81.16% <ø> (-0.02%)	⬇️

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[Tau-J]

Suggested change

	- **What's next? Join the rank of <span style="color:blue"> *MMPose contributors* </span> by creating a new project**!
	- **[:bulb:PETR](./petr)**: End-to-End Multi-Person Pose Estimation with Transformers
	<div align=center>
	<img src="https://github.com/open-mmlab/mmpose/assets/26127467/ec7eb99d-8b8b-4c0d-9714-0ccd33a4f054" width="800" style="width: 800px; height: 200px; object-fit: cover"/>
	</div><br/>
	- **[:bulb:PETR](./petr)**: End-to-End Multi-Person Pose Estimation with Transformers
	<div align=center>
	<img src="https://github.com/open-mmlab/mmpose/assets/26127467/ec7eb99d-8b8b-4c0d-9714-0ccd33a4f054" width="800" style="width: 800px; height: 200px; object-fit: cover"/>
	</div><br/>
	- **What's next? Join the rank of <span style="color:blue"> *MMPose contributors* </span> by creating a new project**!