[Feature Request] Using `optional` and `override` at the same time in defaults

[huterguier]

🚀 Feature Request

Using optional and override at the same time in defaults.

Motivation

There exists no way to optionally override a config in hydra if and only if the optional config file exists.

Pitch

Add the option to use both optionaland override for the same value.

defaults:
  - dataset: imagenet
  - model: alexnet
  - optional override model: ${dataset}/${model}

This makes it possible to select custom version of alexnet based on the dataset used.

I have tried a lot of different approaches to achieve the same goal without any success.