Update type hints

[AdeelH]

Overview

This PR updates type hints and improves docstrings.

Changes include:

• Replace deprecated aliases: Callable, Dict, List, Set, Tuple, Type, with the recommended replacements
• Replace Optional[x] with x | None
• Replace Union[x, y, ...] with x | y | ...
• Add signature to Callable type hints where feasible
• Use Self where applicable
• Remove argument type hints from docstrings (non-exhaustive)
• Fix/improve docstrings

Checklist

• Added unit tests, if applicable
• Updated documentation, if applicable
• Added needs-backport label if the change should be back-ported to the previous release
• PR has a name that won't get you publicly shamed for vagueness

Notes

N/A

Testing Instructions

N/A

[codecov]

Codecov Report

Attention: Patch coverage is 99.45455% with 3 lines in your changes missing coverage. Please review.

Project coverage is 90.09%. Comparing base (8f9f6d4) to head (02c9ffd).
Report is 40 commits behind head on master.

Files with missing lines	Patch %	Lines
...ervision_core/rastervision/core/data/utils/misc.py	80.00%	1 Missing ⚠️
...vision/pytorch_learner/object_detection_learner.py	80.00%	1 Missing ⚠️
...earner/rastervision/pytorch_learner/utils/utils.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2192   +/-   ##
=======================================
  Coverage   90.08%   90.09%           
=======================================
  Files         198      198           
  Lines        9877     9879    +2     
=======================================
+ Hits         8898     8900    +2     
  Misses        979      979           

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