support adding alias for models without hf repo/sku entry (#481)

# What does this PR do?

adds a new method build_model_alias_with_just_llama_model which is
needed for cases like ollama's quantized models which do not really have
a repo in hf and an entry in SKU list.


## Test Plan

pytest -v -s -m "ollama"
llama_stack/providers/tests/inference/test_text_inference.py

---------

Co-authored-by: Dinesh Yeduguru <dineshyv@fb.com>

llama_stack/providers/utils/inference/model_registry.py

@@ -36,6 +36,16 @@ def build_model_alias(provider_model_id: str, model_descriptor: str) -> ModelAli
     )
 
 
+def build_model_alias_with_just_provider_model_id(
+    provider_model_id: str, model_descriptor: str
+) -> ModelAlias:
+    return ModelAlias(
+        provider_model_id=provider_model_id,
+        aliases=[],
+        llama_model=model_descriptor,
+    )
+
+
 class ModelRegistryHelper(ModelsProtocolPrivate):
     def __init__(self, model_aliases: List[ModelAlias]):
         self.alias_to_provider_id_map = {}