Allow specifying extra args for default model in ModelConfig

[AdeelH]

Overview

This PR adds an extra_args field to ModelConfig which accepts a dict. In each ModelConfig subclass, these args are passed as keyword args to the torchvision function used to construct the default model.

This is mainly motivated by the FasterCNN used in OD which accepts a large number of keyword args that can be important but are too many to be added as individual fields to ObjectDetectionModelConfig.

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 test.

[codecov]

Codecov Report

Merging #1713 (9b5b493) into master (40afafc) will decrease coverage by 0.04%.
The diff coverage is 81.81%.

❗ Current head 9b5b493 differs from pull request most recent head a6981b3. Consider uploading reports for the commit a6981b3 to get more accurate results

@@            Coverage Diff             @@
##           master    #1713      +/-   ##
==========================================
- Coverage   75.74%   75.71%   -0.04%     
==========================================
  Files         192      192              
  Lines        9314     9309       -5     
==========================================
- Hits         7055     7048       -7     
- Misses       2259     2261       +2     

Impacted Files	Coverage Δ
...pytorch_learner/object_detection_learner_config.py	84.78% <ø> (ø)
...ch_learner/semantic_segmentation_learner_config.py	90.00% <ø> (ø)
...ision/pytorch_learner/regression_learner_config.py	82.10% <71.42%> (-0.51%)	⬇️
...n/pytorch_learner/classification_learner_config.py	93.84% <100.00%> (+0.09%)	⬆️
...ner/rastervision/pytorch_learner/learner_config.py	82.16% <100.00%> (+0.03%)	⬆️
...data/vector_source/geojson_vector_source_config.py	85.71% <0.00%> (-3.76%)	⬇️
...n/core/data/vector_source/geojson_vector_source.py	86.66% <0.00%> (-3.34%)	⬇️
...ision_core/rastervision/core/data/utils/geojson.py	94.92% <0.00%> (-0.73%)	⬇️

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