Use inference APIs for running llama guard

Test Plan:

First, start a TGI container with `meta-llama/Llama-Guard-3-8B` model
serving on port 5099. See https://github.com/meta-llama/llama-stack/pull/53 and its
description for how.

Then run llama-stack with the following run config:

```
image_name: safety
docker_image: null
conda_env: safety
apis_to_serve:
- models
- inference
- shields
- safety
api_providers:
  inference:
    providers:
    - remote::tgi
  safety:
    providers:
    - meta-reference
  telemetry:
    provider_id: meta-reference
    config: {}
routing_table:
  inference:
  - provider_id: remote::tgi
    config:
      url: http://localhost:5099
      api_token: null
      hf_endpoint_name: null
    routing_key: Llama-Guard-3-8B
  safety:
  - provider_id: meta-reference
    config:
      llama_guard_shield:
        model: Llama-Guard-3-8B
        excluded_categories: []
        disable_input_check: false
        disable_output_check: false
      prompt_guard_shield: null
    routing_key: llama_guard
```

Now simply run `python -m llama_stack.apis.safety.client localhost
<port>` and check that the llama_guard shield calls run correctly. (The
injection_shield calls fail as expected since we have not set up a
router for them.)

llama_stack/providers/registry/safety.py

@@ -15,13 +15,15 @@ def available_providers() -> List[ProviderSpec]:
             api=Api.safety,
             provider_id="meta-reference",
             pip_packages=[
-                "accelerate",
                 "codeshield",
-                "torch",
                 "transformers",
+                "torch --index-url https://download.pytorch.org/whl/cpu",
             ],
             module="llama_stack.providers.impls.meta_reference.safety",
             config_class="llama_stack.providers.impls.meta_reference.safety.SafetyConfig",
+            api_dependencies=[
+                Api.inference,
+            ],
         ),
         remote_provider_spec(
             api=Api.safety,