[Fix] Fix _flip_keypoint of RandomFlip returns the wrong result

[zhouzaida]

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Motivation

resolves #2526

_flip_keypoint of RandomFlip returns the wrong result. _flip_keypoint should concatenate the flipped rather than input keypoints.

Modification

_flip_keypoint of RandomFlip

BC-breaking (Optional)

No

Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here, and update the documentation.

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 test to ensure the correctness.
• The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with some of those projects, like MMDet or MMCls.
• CLA has been signed and all committers have signed the CLA in this PR.

[ly015]

LGTM

[codecov]

Codecov Report

Base: 66.34% // Head: 66.34% // No change to project coverage 👍

Coverage data is based on head (d235bdf) compared to base (4ae327f).
Patch coverage: 100.00% of modified lines in pull request are covered.

❗ Current head d235bdf differs from pull request most recent head 6f82596. Consider uploading reports for the commit 6f82596 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##              2.x    #2527   +/-   ##
=======================================
  Coverage   66.34%   66.34%           
=======================================
  Files         120      120           
  Lines        7746     7746           
  Branches     1074     1074           
=======================================
  Hits         5139     5139           
  Misses       2438     2438           
  Partials      169      169           

Flag	Coverage Δ
unittests	66.34% <100.00%> (ø)

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

Impacted Files	Coverage Δ
mmcv/transforms/processing.py	94.67% <100.00%> (ø)

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