Overwriting hydra configuration groups from CLI

From the CLI, I am trying to overwrite a group of parameters. The structure of my conf is:

conf
├── config.yaml
├── optimizer
│   ├── adamw.yaml
│   ├── adam.yaml
│   ├── default.yaml
│   └── sgd.yaml
├── task
│   ├── default.yaml
│   └── nlp
│       ├── default_seq2seq.yaml
│       ├── summarization.yaml
│       └── text_classification.yaml

task/default.yaml:

# @package task
defaults:
  - _self_
  - /optimizer/adam@cfg.optimizer

_target_: src.core.task.Task
_recursive_: false

cfg:
  prefix_sep: ${training.prefix_sep}

optimizer/default.yaml:

_target_: null
lr: ${training.lr}
weight_decay: 0.001
no_decay:
  - bias
  - LayerNorm.weight

optimizer/adam.yaml:

defaults:
  - default

_target_: torch.optim.Adam

This results in the output config:

task:
  _target_: src.task.nlp.nli_generation.task.NLIGenerationTask
  _recursive_: false
  cfg:
    prefix_sep: ${training.prefix_sep}
    optimizer:
      _target_: torch.optim.Adam
      lr: ${training.lr}
      weight_decay: 0.001
      no_decay:
      - bias
      - LayerNorm.weight

I would like to be able to modify the optimizer via the CLI (say, use sgd), but I am not sure how to achieve this. I tried, but I understand why it fails, this

python train.py task.cfg.optimizer=sgd # fails
python train.py task.cfg.optimizer=/optimizer/sgd #fails

Github discussion here.

You can't override default list entries in this form. See this. In particular:

CONFIG : A config to use when creating the output config. e.g. db/mysql, db/mysql@backup.
GROUP_DEFAULT : An overridable config. e.g. db: mysql, db@backup: mysql.

To be able to override a default list entry, you need to define it as a GROUP_DEFAULT. In your case, it might look like

defaults:
  - _self_
  - /optimizer@cfg.optimizer: adam