Recursive instantiation

this would go well with a change to the config definition of an instantiated object:

Current

model:
  cls: foo.bar.ResNet
  params:
    num_layers: 50

New

model:
  _target_: model.ResNet
  num_layers: 50

Once this is implemented, we can describe recursive instantiation like:

trainer:
  _target_: trainer.Trainer
  batch_size: 32
  model:
    _target_: model.ResNet
    num_layers: 50

An remaining open question is how to support passthrough for nested objects.

Currently, passthrough can override individual values:

trainer = instantiate(cfg.trainer, batch_size=64)

a possible approach is for nested objects is to use a dictionary:

trainer = instantiate(cfg.trainer, batch_size=64, overrides={"trainer.model.num_layers": 50})

[omry]

This is coming in Hydra 1.1. I have a question for people who liked this issue (indicating that they would like to have this supported):

I would like to make recursive instantiation the default, but this is a breaking change for people that are actually interested in it because right now they are getting the nested config as a config and they need to manually call instantiate on it. Assuming that there will be an alternative way to instantiate non recursively, is such a breaking change acceptable to folks here?

Like this comment to indicate yes and dislike it to indicate no. Feel free to followup with a discussion.

[omry]

This is now in Hydra master branch. Feedback welcome.