[Bug] Fix DeformConv2d bias error and add tests

[MeowZheng]

fix #939

[codecov]

Codecov Report

Merging #940 (4fb3a5f) into master (3bcc796) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

❗ Current head 4fb3a5f differs from pull request most recent head a4088c0. Consider uploading reports for the commit a4088c0 to get more accurate results

@@            Coverage Diff             @@
##           master     #940      +/-   ##
==========================================
- Coverage   65.67%   65.67%   -0.01%     
==========================================
  Files         149      149              
  Lines        9452     9451       -1     
  Branches     1720     1720              
==========================================
- Hits         6208     6207       -1     
  Misses       2924     2924              
  Partials      320      320              

Flag	Coverage Δ
unittests	65.67% <0.00%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
mmcv/ops/deform_conv.py	61.48% <0.00%> (-0.29%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 3bcc796...a4088c0. Read the comment docs.