Update example plugin __init__.py files to include registry.set_plugin_version() calls

[AdeelH]

Overview

All plugins are required to register themselves in the Registry by calling registry.set_plugin_version(). However, currently the example plugins and docs do not make this clear. This PR fixes this oversight.

This was brought up in #1652.

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 docs build: https://raster-vision--1665.org.readthedocs.build/en/1665/framework/architecture.html#id4

[codecov]

Codecov Report

Merging #1665 (9646afa) into master (95e50f5) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head 9646afa differs from pull request most recent head 1431dea. Consider uploading reports for the commit 1431dea to get more accurate results

@@            Coverage Diff             @@
##           master    #1665      +/-   ##
==========================================
- Coverage   75.69%   75.68%   -0.02%     
==========================================
  Files         192      192              
  Lines        9254     9254              
==========================================
- Hits         7005     7004       -1     
- Misses       2249     2250       +1     

Impacted Files	Coverage Δ
.../rastervision/pipeline_example_plugin1/__init__.py	100.00% <100.00%> (ø)
.../rastervision/pipeline_example_plugin2/__init__.py	100.00% <100.00%> (ø)
...ervision/pytorch_learner/object_detection_utils.py	75.14% <0.00%> (-0.57%)	⬇️

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