**Description**
This PR fixes#1013.
It adds `draft_model` and `n_draft` to the model YAML config in order to
load models with speculative sampling. This should be compatible as well
with grammars.
example:
```yaml
backend: llama
context_size: 1024
name: my-model-name
parameters:
model: foo-bar
n_draft: 16
draft_model: model-name
```
---------
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
