exp save: save workspace state to an experiment ref

dvc exp commit would create an experiment from the current state of the workspace.

Use cases for this command include:

• Not using DVC pipelines but still want to track experiments in a lightweight way. For example, by just having params in params.yaml and metrics in metrics.json (see #6168), DVC can track and compare experiments.
• Code has already been run. The user might have already run code and wants to record it without committing it to their working branch.
• Want to examine the results before deciding whether to save it as an experiment. Users can do dvc repro and do dvc exp commit whenever they want to track the experiment results.
• Enables users to use dvc repro and still use experiments. Some users may prefer this because they are already used to dvc repro, and in other cases dvc exp run might not work quite the same way. This eases the need to have dvc repro and dvc exp run behave exactly the same (see https://github.com/iterative/dvc.org/issues/2861).

[shcheklein]

One minor thought on this. When I initially read about this I thought about dvc commit + git commit semantics. While this is more like dvc exp stash? or dvc exp save, right? I guess, my concern is that commit is so overloaded that might confuse people.

[dberenbaum]

One minor thought on this. When I initially read about this I thought about dvc commit + git commit semantics. While this is more like dvc exp stash? or dvc exp save, right? I guess, my concern is that commit is so overloaded that might confuse people.

Hmm, agreed. I like dvc exp save. It’s less tangled in git lingo and more obvious to users.