Allow models to be registered as long as llama model is provided (#472)

This PR allows models to be registered with provider as long as the user
specifies a llama model, even though the model does not match our
prebuilt provider specific mapping.
Test:
pytest -v -s
llama_stack/providers/tests/inference/test_model_registration.py -m
"together" --env TOGETHER_API_KEY=<KEY>

---------

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

llama_stack/providers/utils/inference/__init__.py

@@ -31,3 +31,8 @@ def supported_inference_models() -> List[str]:
             or is_supported_safety_model(m)
         )
     ]
+
+
+ALL_HUGGINGFACE_REPOS_TO_MODEL_DESCRIPTOR = {
+    m.huggingface_repo: m.descriptor() for m in all_registered_models()
+}