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/meta-reference-gpu/run.yaml

@@ -19,6 +19,9 @@ providers:
       model: ${env.INFERENCE_MODEL}
       max_seq_len: 4096
       checkpoint_dir: ${env.INFERENCE_CHECKPOINT_DIR:null}
+  - provider_id: sentence-transformers
+    provider_type: inline::sentence-transformers
+    config: {}
   memory:
   - provider_id: faiss
     provider_type: inline::faiss
@@ -77,6 +80,13 @@ models:
   model_id: ${env.INFERENCE_MODEL}
   provider_id: meta-reference-inference
   provider_model_id: null
+  model_type: llm
+- metadata:
+    embedding_dimension: 384
+  model_id: all-MiniLM-L6-v2
+  provider_id: sentence-transformers
+  provider_model_id: null
+  model_type: embedding
 shields: []
 memory_banks: []
 datasets: []