Add new resolver oc.create

[odelalleau]

Motivation

A new resolver oc.create would make it easier to dynamically generate config objects from lists and dictionaries.

For instance imagine you have the following JSON file model.json with the following content:

{"name": "resnet", "n_units_per_layer": [100, 1000, 500]}

You want to load this JSON within an existing config. This could be achieved with:

import json
from omegaconf import DictConfig, OmegaConf

# Define a custom resolver to load JSON files.

def load_json(path):
    with open(path) as f:
        return json.load(f)

OmegaConf.register_new_resolver("load_json", load_json)

# Combine it with `oc.create` to generate a DictConfig with the content of this file.

cfg = OmegaConf.create({"model": "${oc.create:${load_json:'model.json'}}"})
assert isinstance(cfg.model, DictConfig)
assert cfg.model.n_units_per_layer[1]  == 1000

If oc.create wasn't used, then cfg.model would be a plain dictionary that wouldn't support all DictConfig features (like accessing a field with the syntax cfg.model.n_units_per_layer)

Alternatives

It is possible for custom resolvers to explicitly call OmegaConf.create(..., parent=_parent_) to always generate config nodes. The objective of introducing oc.create is (i) to make this more straightforward and (ii) to add more flexibility, so that config nodes are only generated when explicitly requested (there are situations where keeping plain dicts / lists may be preferred).

Additional context

• In current master (d93905e) when a resolver outputs a dict/list, it is automatically converted into a DictConfig / ListConfig. However this may not always be a good thing and will be reverted when introducing oc.create (see discussion: Introduce new oc.env and oc.decode resolvers #606 (comment))