Nicer logging of list of dicts for hyper parameters

[vork]

What does this PR do?

Currently, when hyper parameters are logged with log_hyperparams the function calls _flatten_dict to collapse the dict to a single level. However, when the config contains a list of dicts, this gets flattened to a single string. Instead I would propose to log the list as [key/0/item, key/1/item] etc.

Fixes #19957

Before submitting

• Was this discussed/agreed via a GitHub issue? (not for typos and docs)
  Only a minor change
• 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)
  This does not require any changes just makes the hyper parameter logging slightly nicer
• 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 list all the breaking changes introduced by this pull request?
• Did you update the CHANGELOG? (not for typos, docs, test updates, or minor internal changes/refactors)

PR review

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

Reviewer checklist

• 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

---

📚 Documentation preview 📚: https://pytorch-lightning--19963.org.readthedocs.build/en/19963/

[jammm]

@lantiga @Borda @tchaton @justusschock it would be great if you can take a look at this PR ^^