Misc object detection fixes and improvements

[AdeelH]

Overview

This PR makes multiple small fixes and improvements to object detection-related code. The most notable change is that it is now possible to use an img_sz that is not equal to the chip_sz during prediction.

Checklist

• Added needs-backport label if PR is bug fix that applies to previous minor release
• Ran scripts/format_code and committed any changes
• Documentation updated if needed
• PR has a name that won't get you publicly shamed for vagueness

Notes

N/A

Testing Instructions

See new unit tests.

[codecov]

Codecov Report

Merging #1711 (f4f1ca4) into master (186e4f6) will increase coverage by 0.07%.
The diff coverage is 92.68%.

@@            Coverage Diff             @@
##           master    #1711      +/-   ##
==========================================
+ Coverage   75.62%   75.69%   +0.07%     
==========================================
  Files         192      192              
  Lines        9277     9304      +27     
==========================================
+ Hits         7016     7043      +27     
  Misses       2261     2261              

Impacted Files	Coverage Δ
...ision/core/evaluation/classification_evaluation.py	90.69% <ø> (ø)
...sion/core/evaluation/object_detection_evaluator.py	75.00% <ø> (+8.33%)	⬆️
...vision/pytorch_backend/pytorch_object_detection.py	42.10% <ø> (ø)
...data/label_store/object_detection_geojson_store.py	62.96% <66.66%> (+0.46%)	⬆️
...rvision/core/data/label/object_detection_labels.py	93.91% <80.00%> (-0.57%)	⬇️
...vision/pytorch_learner/object_detection_learner.py	92.20% <92.85%> (+1.43%)	⬆️
...data/label_source/object_detection_label_source.py	55.22% <100.00%> (+0.67%)	⬆️
...n_core/rastervision/core/data/label_store/utils.py	82.60% <100.00%> (+4.83%)	⬆️
...ytorch_learner/dataset/object_detection_dataset.py	46.55% <100.00%> (+0.46%)	⬆️
...ervision/pytorch_learner/object_detection_utils.py	76.79% <100.00%> (+1.65%)	⬆️
... and 1 more

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