torchtune as an optional dependency: Lazy Import

### 🚀 The feature, motivation and pitch We would like to make `torchtune` an optional dependency. The first step towards that is to avoid importing `torchtune` unless it is actively used. To make this migration easier, let's move the top level imports into the functions/classes that require them. > We explicitly acknowledge that this initial step isn't a best practice, but will make the following work simpler. Here's an example where we delay imports here: https://github.com/pytorch/torchchat/blob/1384f7d3d7af0847d8364fe7b300a8b49f2213c2/torchchat/usages/eval.py#L216-L225 **Task:** Update all imports of `torchtune` in the repo, such that imports are only done when necessary - Suggestion: Feel free to send out PR's that only update a subset of all use cases To test your changes, run: - With torchtune installed: https://github.com/pytorch/torchchat/blob/main/docs/multimodal.md#generation - With torchtune uninstalled: `python torchchat.py generate llama3.2-1B` ### Alternatives _No response_ ### Additional context _No response_ ### RFC (Optional) _No response_