Do not instantiate parts of the system that are unnecessary when performing a stage

[ibro45]

When in val, test, or predict mode, optimizers and schedulers aren't necessary.

When running training, no need for test or predict parts. And so on.

[surajpaib]

Revisiting this, I think we can easily do it with the _disabled_ flag now in the config

[ibro45]

I had tried to do it a month ago or so, but was running into the problem of referencing/interpolating config values from the disabled parts if I remember correctly.

Also, with the latest monai version, % referencing seems to work even less than it did. I should really open an issue about why % (referencing a config item as-is) is not as usable as @ (referencing a config item that is resolved, e.g., instantiated or code executed).

Mentioning this because you should be able to %-reference a disabled config item. Obviously, @-referencing should not work for the disabled config item because it's not being resolved.

[ibro45]

See if this can be handled nicely when we'll be switching to the BundleWorkflows