[Fix] Fix det demo bug

[Wuziyi616]

Fix #596

[codecov]

Codecov Report

Merging #600 (aa5e9e0) into master (e65772d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #600   +/-   ##
=======================================
  Coverage   50.86%   50.86%           
=======================================
  Files         197      197           
  Lines       15067    15067           
  Branches     2453     2453           
=======================================
  Hits         7664     7664           
  Misses       6893     6893           
  Partials      510      510           

Flag	Coverage Δ
unittests	50.86% <ø> (ø)

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Impacted Files	Coverage Δ
mmdet3d/apis/inference.py	53.62% <ø> (ø)

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