What does this PR do?

Log gain as a metric on progress bar, so that LR scheduler also sees it.
Remove the renaming mechanism loss->gain.
(Our outdated PL displays loss in running mean and the raw gain, but newer PL will hide loss.)
Change to a more natural config name: disable -> enable.

Type of change

Please check all relevant options.

[x] Improvement (non-breaking)
[ ] Bug fix (non-breaking)
[ ] New feature (non-breaking)
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
[ ] This change requires a documentation update

Please describe the tests that you ran to verify your changes. Consider listing any relevant details of your test configuration.

[x] The title is self-explanatory and the description concisely explains the PR
[x] My PR does only one thing, instead of bundling different changes together
[ ] I list all the breaking changes introduced by this pull request
[x] I have commented my code
[ ] I have added tests that prove my fix is effective or that my feature works
[x] New and existing unit tests pass locally with my changes
[x] I have run pre-commit hooks with pre-commit run -a command without errors

Make sure you had fun coding 🙃