[Evaluation] - Support multiple splits

[JuanSeBestia]

Feature Request

I want to obtain the evaluation of different datasets of a single job
lumi eval -h && lumi eval -c config_ssd.yml --split valid --split train --split test --watch --from-global-step 0

I understand that it is not so trivial because it was being kept as a summary of work scalars like this

sounds very crazy?

[dekked]

It's doable, and not crazy, to support multiple splits!

This looks to be a good first issue for contributing 🕺