[Feat] Add KeypointEndPointError

[LareinaM]

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Motivation

Add KeypointEndPointError metric.

Modification

1. Add mmeval/metrics/keypoint_epe.py
2. Modify metric doc and __init__.py
3. Add unit test tests/test_metrics/test_keypoint_epe.py

Related PR on MMPose

open-mmlab/mmpose#1996
Update EPE to use MMEval

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.

[C1rN09]

The lint failed. Generally good! ^_^

[LareinaM]

The lint failed. Generally good! ^_^

I looked into the lint and found it failed in the mdformat phase. Seems not very closely related to this PR (because I have checked that the changed files pass this hook)?

[C1rN09]

The lint failed. Generally good! ^_^

I looked into the lint and found it failed in the mdformat phase. Seems not very closely related to this PR (because I have checked that the changed files pass this hook)?

I checked with pre-commit run --all-files and it actually failed linting. You can have a try