Fix detection ddp spawn

[ethanwharris]

What does this PR do?

Fixes #1219

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[codecov]

Codecov Report

Merging #1222 (062cd15) into master (e943921) will decrease coverage by 0.02%.
The diff coverage is 67.44%.

@@            Coverage Diff             @@
##           master    #1222      +/-   ##
==========================================
- Coverage   91.11%   91.09%   -0.03%     
==========================================
  Files         284      284              
  Lines       12754    12768      +14     
==========================================
+ Hits        11621    11631      +10     
- Misses       1133     1137       +4     

Flag	Coverage Δ
unittests	91.09% <67.44%> (-0.03%)	⬇️

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

Impacted Files	Coverage Δ
flash/image/detection/backbones.py	74.41% <18.18%> (-3.09%)	⬇️
flash/core/integrations/icevision/transforms.py	91.44% <69.23%> (-1.80%)	⬇️
flash/core/integrations/icevision/adapter.py	93.24% <83.33%> (+0.09%)	⬆️
flash/core/integrations/icevision/backbones.py	94.44% <100.00%> (+1.11%)	⬆️
flash/text/question_answering/model.py	92.56% <0.00%> (+0.67%)	⬆️

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