Docs finetuning callback example (#21216)

* add to documentation

* improve docstring

* fix docs

* skip doctest if torchvision missing

* missing space

* changelog

* fix mistake

* Empty-Commit

* empty commit to rerun ci

* try fixing doctest

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Co-authored-by: Jirka Borovec <6035284+Borda@users.noreply.github.com>
Co-authored-by: jirka <jirka.borovec@seznam.cz>
Co-authored-by: Deependu Jha <deependujha21@gmail.com>