Add serve support for object detection

[ethanwharris]

What does this PR do?

Fixes #1279

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[codecov]

Codecov Report

Merging #1370 (40dbcb3) into master (323a044) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1370   +/-   ##
=======================================
  Coverage   92.77%   92.77%           
=======================================
  Files         285      285           
  Lines       12671    12685   +14     
=======================================
+ Hits        11755    11768   +13     
- Misses        916      917    +1     

Flag	Coverage Δ
unittests	92.77% <100.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
flash/core/integrations/icevision/transforms.py	92.30% <100.00%> (+0.23%)	⬆️
flash/core/utilities/imports.py	91.42% <100.00%> (ø)
flash/image/detection/model.py	97.29% <100.00%> (+0.86%)	⬆️
flash/image/instance_segmentation/data.py	100.00% <100.00%> (ø)
flash/image/keypoint_detection/data.py	97.72% <100.00%> (ø)
flash/core/serve/dag/task.py	96.82% <0.00%> (-1.06%)	⬇️
flash/text/question_answering/model.py	93.87% <0.00%> (+0.68%)	⬆️

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