dvc repro: only download the necessary artefacts to re-run the pipeline

[ettadar]

Let's say there is a 3 stages workflow: A -> B -> C:

• A run a.py and produce a.json,
• B run b.py that takes a.json as input and produce b.json,
• C run c.py that takes b.json as input and produce c.json.

It would be very handy to have a flag in dvc repro to download only what is necessary to rerun the stages that changed. For example, when working on a fresh clone of the repo:

• if I change nothing and run dvc repro --new-flag -> dvc download nothing and run nothing.
• if I change c.py and run dvc repro --new-flag -> dvc only download b.json and run step C.
• if I change b.py and run dvc repro --new-flag -> dvc download a.json and run steps B and C.

This become particularly useful when working with big pipelines that train multiple models. Downloading all the training data for all the model can takes a lot of time and a lot of space on the disk.

[dberenbaum]

Seems like a duplicate of #4742, so I'll transfer the comment over there.