add embedding model by default to distribution templates (#617)

# What does this PR do?
Adds the sentence transformer provider and the `all-MiniLM-L6-v2`
embedding model to the default models to register in the run.yaml for
all providers.

## Test Plan
llama stack build --template together --image-type conda
llama stack run
~/.llama/distributions/llamastack-together/together-run.yaml

llama_stack/templates/tgi/run-with-safety.yaml

@@ -79,10 +79,12 @@ models:
   model_id: ${env.INFERENCE_MODEL}
   provider_id: tgi-inference
   provider_model_id: null
+  model_type: llm
 - metadata: {}
   model_id: ${env.SAFETY_MODEL}
   provider_id: tgi-safety
   provider_model_id: null
+  model_type: llm
 shields:
 - params: null
   shield_id: ${env.SAFETY_MODEL}