add completion api support to nvidia inference provider (#533)

# What does this PR do?

add the completion api to the nvidia inference provider


## Test Plan

while running the meta/llama-3.1-8b-instruct NIM from
https://build.nvidia.com/meta/llama-3_1-8b-instruct?snippet_tab=Docker

```
➜ pytest -s -v --providers inference=nvidia llama_stack/providers/tests/inference/ --env NVIDIA_BASE_URL=http://localhost:8000 -k test_completion --inference-model Llama3.1-8B-Instruct
=============================================== test session starts ===============================================
platform linux -- Python 3.10.15, pytest-8.3.3, pluggy-1.5.0 -- /home/matt/.conda/envs/stack/bin/python
cachedir: .pytest_cache
rootdir: /home/matt/Documents/Repositories/meta-llama/llama-stack
configfile: pyproject.toml
plugins: anyio-4.6.2.post1, asyncio-0.24.0, httpx-0.34.0
asyncio: mode=strict, default_loop_scope=None
collected 20 items / 18 deselected / 2 selected                                                                             

llama_stack/providers/tests/inference/test_text_inference.py::TestInference::test_completion[-nvidia] PASSED
llama_stack/providers/tests/inference/test_text_inference.py::TestInference::test_completion_structured_output[-nvidia] SKIPPED

============================= 1 passed, 1 skipped, 18 deselected, 6 warnings in 5.40s =============================
```

the structured output functionality works but the accuracy fails

## Before submitting

- [ ] This PR fixes a typo or improves the docs (you can dismiss the
other checks if that's the case).
- [x] Ran pre-commit to handle lint / formatting issues.
- [x] Read the [contributor
guideline](https://github.com/meta-llama/llama-stack/blob/main/CONTRIBUTING.md),
      Pull Request section?
- [ ] Updated relevant documentation.
- [x] Wrote necessary unit or integration tests.

llama_stack/providers/tests/inference/test_text_inference.py

@@ -94,6 +94,7 @@ class TestInference:
             "remote::tgi",
             "remote::together",
             "remote::fireworks",
+            "remote::nvidia",
             "remote::cerebras",
         ):
             pytest.skip("Other inference providers don't support completion() yet")
@@ -129,9 +130,7 @@ class TestInference:
 
     @pytest.mark.asyncio
     @pytest.mark.skip("This test is not quite robust")
-    async def test_completions_structured_output(
-        self, inference_model, inference_stack
-    ):
+    async def test_completion_structured_output(self, inference_model, inference_stack):
         inference_impl, _ = inference_stack
 
         provider = inference_impl.routing_table.get_provider_impl(inference_model)
@@ -140,6 +139,7 @@ class TestInference:
             "remote::tgi",
             "remote::together",
             "remote::fireworks",
+            "remote::nvidia",
             "remote::vllm",
             "remote::cerebras",
         ):