Add support for from_files to ObjectDetectionData

[ethanwharris]

What does this PR do?

Fixes #1069

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[codecov]

Codecov Report

Merging #1154 (ea1e5cd) into master (450902d) will increase coverage by 0.01%.
The diff coverage is 90.16%.

@@            Coverage Diff             @@
##           master    #1154      +/-   ##
==========================================
+ Coverage   89.23%   89.24%   +0.01%     
==========================================
  Files         286      286              
  Lines       13022    13041      +19     
==========================================
+ Hits        11620    11639      +19     
  Misses       1402     1402              

Flag	Coverage Δ
unittests	89.24% <90.16%> (+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	93.24% <82.85%> (+0.04%)	⬆️
flash/core/data/io/classification_input.py	100.00% <100.00%> (ø)
flash/core/data/utilities/classification.py	98.22% <100.00%> (+0.03%)	⬆️
flash/image/detection/data.py	100.00% <100.00%> (ø)
flash/image/detection/input.py	100.00% <100.00%> (ø)

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