[Feature Request] Reuse instantiated object in multiple locations

[nikhilxb]

🚀 Feature Request

I would like to reuse a single instantiated object in multiple locations in a config.

Motivation

Is your feature request related to a problem? Please describe.

Here is an example in machine learning: I would like model.encoder1 == model.encoder2 so that they can share weights. However, the below config would result in 2 different instances (with different weights).

model:
  _target_: models.MyModel
  encoder1:
    _target_: models.MyEncoder
  encoder2:
    _target_: models.MyEncoder

More generally, there are times when I want to configure a deeply nested object where certain nodes in the tree are shared.

Pitch

Describe the solution you'd like
Maybe something along the lines of:

model:
  _target_: models.MyModel
  encoder1:
    _target_: models.MyEncoder
  encoder2: ${object:model.encoder1}

It seems that with just the default variable interpolation, the config is interpolated before instantiation, resulting in 2 different instances.

Describe alternatives you've considered

• Rewriting MyModel: that works in this simple case, but there are complex/more nested cases in which I want to share modules. Rewriting it all would defeat the modularity that is the benefit of using Hydra. Sometimes I will try both cases: shared weights and non-shared weights.

Are you willing to open a pull request? (See CONTRIBUTING)
Maybe, but I'm unfamiliar with the codebase and Hydra internals, so I wouldn't trust myself unless it's an easy fix.

[omry]

Thanks. This is a dup of #1393.