[REQUEST] Per-model sampler override presets

[frost555]

Problem

sampling.override_preset is global: every model shares one
preset. Models with different sampling parameters (e.g. Qwen 3.8 wants top_k 20,
Gemma 4 wants top_k 64) can't be served using inline_model_loading: true.

Solution

Optional per-model preset via sampler_preset in the model directory's
tabby_config.yml (with the same fallback chain as other per-model options):

# models/qwen3.8-27b/tabby_config.yml
model:
  sampler_preset: qwen3.8-27b

# models/gemma4/tabby_config.yml
model:
  sampler_preset: gemma4

Resolution: request params > model preset > global preset > built-in defaults.