Conform to new torchvision API for specifying pretrained weights

[AdeelH]

Overview

In version 0.13, TorchVision introduced a new API for specifying pretrained weights and added a deprecation warning for the old API. This PR updates the instances of the use of the old API within Raster Vision to the new API.

This is entirely an internal change and does not affect the ModelConfig API, which still accepts the pretrained field.

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

• There should no longer be deprecation warnings when creating models.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@6b44624). Click here to learn what that means.
The diff coverage is 100.00%.

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  Files             ?      187           
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Impacted Files	Coverage Δ
...n/pytorch_learner/classification_learner_config.py	95.38% <100.00%> (ø)
...pytorch_learner/object_detection_learner_config.py	86.66% <100.00%> (ø)
...ision/pytorch_learner/regression_learner_config.py	88.29% <100.00%> (ø)
...ch_learner/semantic_segmentation_learner_config.py	92.13% <100.00%> (ø)