chore: update default model config settings

[johnnygreco]

This PR updates the default Nemotron 3 Nano inference parameters to what the team tuned it to: temp=1, top_p=1.

I also updated how we store default model settings. The updates make it so

• All the default settings are in one file (constants.py).
• We can easily override default settings when necessary.

sanity check comparing old vs new default config file:

➜ diff  ~/.data-designer/model_configs.yaml ~/Desktop/model_configs.yaml
9,10c9,10
<     temperature: 1.0
<     top_p: 1.0
---
>     temperature: 0.85
>     top_p: 0.95

[johnnygreco]

main update is to keep the default models next to their default settings

[nabinchha]

🚢