exp run: Separate exp param parsing from other commands

[sjawhar]

• ❗ I have followed the Contributing to DVC checklist.

• 📖 If this PR requires documentation updates, I have created a separate PR (or issue, at least) in dvc.org and linked it here.

Thank you for the contribution - we'll try to review it as soon as possible. 🙏

Purpose

Will close #5302

Approach

Since dvc exp run is the only command that uses parameter overrides (as compared with dvc run, where you are simply specifying a list of parameters), it makes sense for it have a separate parsing function. Conveniently, dvc.utils.cli_parse.loads_params is only being used by dvc.repo.experiments.run.run. So I renamed this function to loads_param_overrides for clarity and updated it to use the logic described in the source issue. I also updated the test to reflect the new possibilities.

I didn't see anywhere in the docs where dvc experiments is documented, otherwise I would open a PR there as well.

[pmrowla]

LGTM, thanks for the PR!

Regarding the docs, there's no live documentation for experiments yet since 2.0 is still unreleased. There's an open docs PR that I can update once this is merged.

[dberenbaum]

LGTM! Thanks, @sjawhar

[pmrowla]

Thanks @sjawhar!