Add mypy workflow check and fix all mypy-found bugs

[lboegner]

Summary

Added mypy to GitHub workflows as a static type checker to raise code quality and safety bars. Fixed all mypy errors throughout codebase (type safety and actual bugs caught by mypy). Formatted using pyfmt. Added a PR template to guide developers towards writing and checking quality code locally prior to submitting PRs in a reproducible manner.

Test Plan

Ran suite of checks in list below.

Before Submitting

• [X] Check mypy locally
  • pip3 install mypy==1.2.0
  • mypy --ignore-missing-imports torchsig
  • Address any error messages
• [X] Lint check locally
  • pip3 install flake8
  • flake8 --select=E9,F63,F7,F82 torchsig
  • Address any error messages
• [X] Run formatter if needed
  • pip3 install git+https://github.com/GooeeIOT/pyfmt.git
  • pyfmt torchsig
• [X] Run test suite locally
  • pytest --ignore-glob=*_figures.py --ignore-glob=*_benchmark.py
  • Ensure tests are successful prior to submitting PR