[Feature Request] Allow partial instantiation with factories

[hobbeshunter]

🚀 Feature Request

Add some way to not instantiate the whole config tree but stop when parameters are missing and create a factory (that needs to receive the missing parameters) instead.

Motivation

I have a config tree with many target objects that I instantiate with hydra. However, sometimes it happens that a class has both: parameters that are config params and parameters that are just known at runtime. So it would be nice if hydra would detect when parameters are missing and return a factory instead.

Pitch

I came up with a hacky solution that requires changes to omegaconf (see hobbeshunter/omegaconf@master...hobbeshunter:omegaconf:non-strict-resolve) and hydra (see hobbeshunter/hydra@main...hobbeshunter:hydra:instantiate-factory).

With a little helper class

class Factory:

    def __init__(self, cfg: DictConfig):
        self._cfg = cfg
        self._cfg._set_flag(flags=["allow_objects", "struct", "readonly"],
                            values=[True, False, False])

    def __call__(self, _name_: str, **kwargs):
        cfg = OmegaConf.merge(self._cfg, kwargs)
        return instantiate(cfg.__getattr__(_name_), _convert_="object")

a config could look like:

_target_: my_package.FactoryReceiver
  factory:
    _target_: hydra.utils.Factory
      _recursive_: false
      cfg:
        a: ???
        instance:
          _target_: my_package.BClass
            a: ${..a}
            b: 5

A FactoryReceiver could e.g. look like

class FactoryReceiver:

    def __init__(self, factory: Factory):
        self._factory = factory

    def do_something(self):
        my_instance = self._factory(_name_="instance", a=7)

As can be seen above FactoryReceiver only has to know that it can instantiate an object named instance by providing an a. Instance can be arbitrarily complex.

IMHO that allows for nice parametrizable object composition.