### Check for existing issues

- [x] I have searched the existing issues and checked that my issue is not a duplicate.

### The Feature

Allow a `model_name` entry to reference other model groups as its members, enabling a "group of groups" or model group composition pattern.

### Motivation, pitch

Currently, if you want a single top-level group (e.g. `fast`) that load-balances across multiple named sub-groups with different providers and rate limits, you have to duplicate every deployment under both the sub-group name and the top-level name. In a real config this adds up fast, in my case, around 1000 lines of duplication.

Example of what's currently required:

```yaml # Sub-group 1: two providers for GPT-OSS-120b - model_name: fast/GPT-OSS-120b litellm_params: model: groq/openai/gpt-oss-120b rpm: 30 order: 2 - model_name: fast/GPT-OSS-120b litellm_params: model: cerebras/gpt-oss-120b rpm: 5 order: 1

# Sub-group 2 - model_name: fast/glm-4.7 litellm_params: model: cerebras/zai-glm-4.7 rpm: 5

# To have a "fast" umbrella you must duplicate everything again: - model_name: fast litellm_params: model: groq/openai/gpt-oss-120b rpm: 30 …