[Feature] Add PoseFormer backbone

[QwQ2000]

Motivation

Modification

Add backbone, head and config of the PoseFormer (ICCV 2021) into the repository.

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[bot]]

Codecov Report

Attention: 17 lines in your changes are missing coverage. Please review.

Files	Coverage Δ
mmpose/models/backbones/__init__.py	100.00% <100.00%> (ø)
mmpose/models/heads/__init__.py	100.00% <100.00%> (ø)
mmpose/datasets/pipelines/pose3d_transform.py	83.78% <66.66%> (+8.45%)	:arrow_up:
mmpose/models/heads/poseformer_head.py	75.00% <75.00%> (ø)
mmpose/models/backbones/poseformer.py	91.58% <91.58%> (ø)

... and 26 files with indirect coverage changes

:loudspeaker: Thoughts on this report? Let us know!

[darcula1993]

what's wrong with this branch? Is ther any update? @QwQ2000 @ly015

[QwQ2000]

Sorry, I have resigned from the Shanghai AI Lab for a long time. I'm busy working on other projects so I don't have much time for this. If someone could help me finish this feature, I will be always willing to help.