Allow ref to unique ID in config files

I’ve found myself in the situation where I’d like to use the results of old steps (like previously trained models) in a new experiment (like to compare those models), but I can’t easily reproduce a config file that would lead to the same unique ID for those old steps. I was thinking that we could also support doing `{type: "ref", ref: "TorchTrainStep-UR29FRimidkJiFBXyoYW2xPkjT8fz4Ha"}`, i.e. where you pass the unique ID of a step as the “ref”, and tango will fetch that from the cache