fix: make _Layer, _InputConversion, _OutputConversion public

[lars-reimann]

Closes #690
Closes #691

Summary of Changes

• Make _Layer public (now called Layer)
• Make _InputConversion public (now called InputConversion)
• Make _OutputConversion public (now called OutputConversion)

[github-actions]

🦙 MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ PYTHON	black	8	0	0	0.84s
✅ PYTHON	mypy	8		0	2.34s
✅ PYTHON	ruff	8	0	0	0.23s
✅ REPOSITORY	git_diff	yes		no	0.27s

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (6909652) to head (ad63ee1).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #692   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           67        67           
  Lines         4782      4782           
=========================================
  Hits          4782      4782           

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[lars-reimann]

🎉 This PR is included in version 0.22.1 🎉

The release is available on:

• v0.22.1
• GitHub release

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