mark OptimizerLoop.backward method protected

[awaelchli]

What does this PR do?

Part of #7938

All methods that are not part of the base loop interface get marked as protected. We want to make these methods protected so custom loop implementations don't rely on them as we will likely push out further refactors around them.

Before submitting

• Was this discussed/approved via a GitHub issue? (not for typos and docs)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests? (not for typos and docs)
• Did you verify new and existing tests pass locally with your changes?
• Did you update the CHANGELOG? (not for typos, docs, test updates, or internal minor changes/refactorings)

PR review

Anyone in the community is free to review the PR once the tests have passed.
Before you start reviewing make sure you have read Review guidelines. In short, see the following bullet-list:

• Is this pull request ready for review? (if not, please submit in draft mode)
• Check that all items from Before submitting are resolved
• Make sure the title is self-explanatory and the description concisely explains the PR
• Add labels and milestones (and optionally projects) to the PR so it can be classified

Did you have fun?

I made sure I had fun coding 🙃

[codecov]

Codecov Report

Merging #9514 (428209e) into master (23450e2) will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #9514   +/-   ##
======================================
  Coverage      89%     89%           
======================================
  Files         180     180           
  Lines       15090   15090           
======================================
  Hits        13404   13404           
  Misses       1686    1686