[Fix] Fix a bug in KNN op

[yezhen17]

No description provided.

[codecov]

Codecov Report

Merging #371 (83f9fb2) into master (f0ba0ce) will not change coverage.
The diff coverage is 0.00%.

❗ Current head 83f9fb2 differs from pull request most recent head 03db9d4. Consider uploading reports for the commit 03db9d4 to get more accurate results

@@           Coverage Diff           @@
##           master     #371   +/-   ##
=======================================
  Coverage   49.55%   49.55%           
=======================================
  Files         177      177           
  Lines       11834    11834           
  Branches     1844     1844           
=======================================
  Hits         5864     5864           
  Misses       5610     5610           
  Partials      360      360           

Flag	Coverage Δ
unittests	49.55% <0.00%> (ø)

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Impacted Files	Coverage Δ
mmdet3d/ops/knn/knn.py	32.14% <0.00%> (ø)

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