[FEATURE] Re-design --no-cache

[hasan7n]

Running medperf benchmark run or medperf run with --no-cache currently doesn't work.

After merging #483 , rerunning an experiment with existing prediction folder will raise an error and ask users to manually delete that folder. --no-cache will simply ignore that there is an existing result but it doesn't ignore that there are existing predictions and hence raises an error.

Before implementing the trivial solution (which is simply overwrite predictions when --no-cache), we should think about:

• an option to proceed directly to results evaluation when predictions are present?
• Is it 'safe' to delete predictions with a simple --no-cache flag? Running models may take days. Perhaps it's better to move them to some .cache and provide users a command to clear cache to save storage.