[Fix] Fix knn multi-gpu bug

[fangyixiao18]

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Motivation

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

Modification

Multi-gpu results and single gpu results

BC-breaking (Optional)

Does the modification introduce changes that break the backward compatibility of the downstream repositories?
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)

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Checklist

Before PR:

• Pre-commit or other linting tools are used to fix the potential lint issues.
• Bug fixes are fully covered by unit tests, the case that causes the bug should be added in the unit tests.
• The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, like docstring or example tutorials.

After PR:

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

[codecov]

Codecov Report

Base: 81.61% // Head: 82.11% // Increases project coverage by +0.50% 🎉

Coverage data is based on head (9297cb9) compared to base (fbe0ff6).
Patch coverage: 12.90% of modified lines in pull request are covered.

❗ Current head 9297cb9 differs from pull request most recent head 2ca9f0b. Consider uploading reports for the commit 2ca9f0b to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           dev-1.x     #634      +/-   ##
===========================================
+ Coverage    81.61%   82.11%   +0.50%     
===========================================
  Files          135      135              
  Lines         4961     4921      -40     
  Branches       672      660      -12     
===========================================
- Hits          4049     4041       -8     
+ Misses         774      743      -31     
+ Partials       138      137       -1     

Flag	Coverage Δ
unittests	82.11% <12.90%> (+0.50%)	⬆️

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

Impacted Files	Coverage Δ
mmselfsup/engine/hooks/deepcluster_hook.py	38.88% <0.00%> (-0.55%)	⬇️
mmselfsup/utils/gather.py	100.00% <ø> (+73.33%)	⬆️
mmselfsup/utils/collect.py	14.63% <8.33%> (-4.88%)	⬇️
mmselfsup/models/utils/extractor.py	53.33% <25.00%> (-0.84%)	⬇️
mmselfsup/utils/__init__.py	100.00% <100.00%> (ø)
mmselfsup/datasets/transforms/processing.py	89.00% <0.00%> (+0.21%)	⬆️

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