Fix minor bugs of computing iou3d

[Tai-Wang]

Fix minor bugs encountered when computing iou3d with gpu while the specified cuda id is not default 0.
Fix #65

[codecov]

Codecov Report

Merging #69 into master will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master      #69   +/-   ##
=======================================
  Coverage   51.37%   51.37%           
=======================================
  Files         139      139           
  Lines        8453     8453           
  Branches     1308     1308           
=======================================
  Hits         4343     4343           
  Misses       3831     3831           
  Partials      279      279           

Flag	Coverage Δ
#unittests	51.37% <0.00%> (ø)

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Impacted Files	Coverage Δ
mmdet3d/ops/iou3d/iou3d_utils.py	27.77% <0.00%> (ø)

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