Model Merge depends on InvokeAI

src/invoke_training/model_merge/scripts/merge_lora_into_model.py L10:

```
# HACK(ryand): Import order matters, because invokeai contains circular imports.
from invokeai.backend.model_manager import BaseModelType
from invokeai.backend.lora import LoRAModelRaw
from invokeai.backend.model_patcher import ModelPatcher
```

But the documentation draft (docs/get-started/installation.md) says:

> Create and activate a python [virtual environment](https://docs.python.org/3/library/venv.html#creating-virtual-environments). This creates an isolated environment for invoke-training and its dependencies that won't interfere with other python environments on your system, including any installations of [InvokeAI](https://www.github.com/invoke-ai/invokeai).

And presently **invokeai** is not listed in pyproject.toml as a dependency.