fix: imshow_bboxes fails with incontiguous img

[LutingWang]

Fix #801

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[codecov]

Codecov Report

Merging #823 (57aae80) into master (4a694cc) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #823      +/-   ##
==========================================
- Coverage   62.26%   62.25%   -0.01%     
==========================================
  Files         144      144              
  Lines        8506     8507       +1     
  Branches     1522     1522              
==========================================
  Hits         5296     5296              
- Misses       2945     2946       +1     
  Partials      265      265              

Flag	Coverage Δ
unittests	62.25% <0.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
mmcv/visualization/image.py	10.76% <0.00%> (-0.17%)	⬇️

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[ycxioooong]

LGTM. @hellock pls have a look.