chore: create OpenAIMixin for inference providers with an OpenAI-compat API that need to implement openai_* methods (#2835)

# What does this PR do?

add an `OpenAIMixin` for use by inference providers who remote endpoints
support an OpenAI compatible API.

use is demonstrated by refactoring
- OpenAIInferenceAdapter
- NVIDIAInferenceAdapter (adds embedding support)
- LlamaCompatInferenceAdapter

## Test Plan

existing unit and integration tests

llama_stack/providers/remote/inference/nvidia/nvidia.py

@@ -7,9 +7,8 @@
 import logging
 import warnings
 from collections.abc import AsyncIterator
-from typing import Any
 
-from openai import APIConnectionError, AsyncOpenAI, BadRequestError, NotFoundError
+from openai import APIConnectionError, BadRequestError
 
 from llama_stack.apis.common.content_types import (
     InterleavedContent,
@@ -28,12 +27,6 @@ from llama_stack.apis.inference import (
     Inference,
     LogProbConfig,
     Message,
-    OpenAIChatCompletion,
-    OpenAIChatCompletionChunk,
-    OpenAICompletion,
-    OpenAIEmbeddingsResponse,
-    OpenAIMessageParam,
-    OpenAIResponseFormatParam,
     ResponseFormat,
     SamplingParams,
     TextTruncation,
@@ -47,8 +40,8 @@ from llama_stack.providers.utils.inference.model_registry import (
 from llama_stack.providers.utils.inference.openai_compat import (
     convert_openai_chat_completion_choice,
     convert_openai_chat_completion_stream,
-    prepare_openai_completion_params,
 )
+from llama_stack.providers.utils.inference.openai_mixin import OpenAIMixin
 from llama_stack.providers.utils.inference.prompt_adapter import content_has_media
 
 from . import NVIDIAConfig
@@ -64,7 +57,20 @@ from .utils import _is_nvidia_hosted
 logger = logging.getLogger(__name__)
 
 
-class NVIDIAInferenceAdapter(Inference, ModelRegistryHelper):
+class NVIDIAInferenceAdapter(OpenAIMixin, Inference, ModelRegistryHelper):
+    """
+    NVIDIA Inference Adapter for Llama Stack.
+
+    Note: The inheritance order is important here. OpenAIMixin must come before
+    ModelRegistryHelper to ensure that OpenAIMixin.check_model_availability()
+    is used instead of ModelRegistryHelper.check_model_availability(). It also
+    must come before Inference to ensure that OpenAIMixin methods are available
+    in the Inference interface.
+
+    - OpenAIMixin.check_model_availability() queries the NVIDIA API to check if a model exists
+    - ModelRegistryHelper.check_model_availability() just returns False and shows a warning
+    """
+
     def __init__(self, config: NVIDIAConfig) -> None:
         # TODO(mf): filter by available models
         ModelRegistryHelper.__init__(self, model_entries=MODEL_ENTRIES)
@@ -88,45 +94,21 @@ class NVIDIAInferenceAdapter(Inference, ModelRegistryHelper):
 
         self._config = config
 
-    async def check_model_availability(self, model: str) -> bool:
+    def get_api_key(self) -> str:
         """
-        Check if a specific model is available.
+        Get the API key for OpenAI mixin.
 
-        :param model: The model identifier to check.
-        :return: True if the model is available dynamically, False otherwise.
+        :return: The NVIDIA API key
         """
-        try:
-            await self._client.models.retrieve(model)
-            return True
-        except NotFoundError:
-            logger.error(f"Model {model} is not available")
-        except Exception as e:
-            logger.error(f"Failed to check model availability: {e}")
-        return False
+        return self._config.api_key.get_secret_value() if self._config.api_key else "NO KEY"
 
-    @property
-    def _client(self) -> AsyncOpenAI:
+    def get_base_url(self) -> str:
         """
-        Returns an OpenAI client for the configured NVIDIA API endpoint.
+        Get the base URL for OpenAI mixin.
 
-        :return: An OpenAI client
+        :return: The NVIDIA API base URL
         """
-
-        base_url = f"{self._config.url}/v1" if self._config.append_api_version else self._config.url
-
-        return AsyncOpenAI(
-            base_url=base_url,
-            api_key=(self._config.api_key.get_secret_value() if self._config.api_key else "NO KEY"),
-            timeout=self._config.timeout,
-        )
-
-    async def _get_provider_model_id(self, model_id: str) -> str:
-        if not self.model_store:
-            raise RuntimeError("Model store is not set")
-        model = await self.model_store.get_model(model_id)
-        if model is None:
-            raise ValueError(f"Model {model_id} is unknown")
-        return model.provider_model_id
+        return f"{self._config.url}/v1" if self._config.append_api_version else self._config.url
 
     async def completion(
         self,
@@ -160,7 +142,7 @@ class NVIDIAInferenceAdapter(Inference, ModelRegistryHelper):
         )
 
         try:
-            response = await self._client.completions.create(**request)
+            response = await self.client.completions.create(**request)
         except APIConnectionError as e:
             raise ConnectionError(f"Failed to connect to NVIDIA NIM at {self._config.url}: {e}") from e
 
@@ -213,7 +195,7 @@ class NVIDIAInferenceAdapter(Inference, ModelRegistryHelper):
             extra_body["input_type"] = task_type_options[task_type]
 
         try:
-            response = await self._client.embeddings.create(
+            response = await self.client.embeddings.create(
                 model=provider_model_id,
                 input=input,
                 extra_body=extra_body,
@@ -228,16 +210,6 @@ class NVIDIAInferenceAdapter(Inference, ModelRegistryHelper):
         #
         return EmbeddingsResponse(embeddings=[embedding.embedding for embedding in response.data])
 
-    async def openai_embeddings(
-        self,
-        model: str,
-        input: str | list[str],
-        encoding_format: str | None = "float",
-        dimensions: int | None = None,
-        user: str | None = None,
-    ) -> OpenAIEmbeddingsResponse:
-        raise NotImplementedError()
-
     async def chat_completion(
         self,
         model_id: str,
@@ -274,7 +246,7 @@ class NVIDIAInferenceAdapter(Inference, ModelRegistryHelper):
         )
 
         try:
-            response = await self._client.chat.completions.create(**request)
+            response = await self.client.chat.completions.create(**request)
         except APIConnectionError as e:
             raise ConnectionError(f"Failed to connect to NVIDIA NIM at {self._config.url}: {e}") from e
 
@@ -283,112 +255,3 @@ class NVIDIAInferenceAdapter(Inference, ModelRegistryHelper):
         else:
             # we pass n=1 to get only one completion
             return convert_openai_chat_completion_choice(response.choices[0])
-
-    async def openai_completion(
-        self,
-        model: str,
-        prompt: str | list[str] | list[int] | list[list[int]],
-        best_of: int | None = None,
-        echo: bool | None = None,
-        frequency_penalty: float | None = None,
-        logit_bias: dict[str, float] | None = None,
-        logprobs: bool | None = None,
-        max_tokens: int | None = None,
-        n: int | None = None,
-        presence_penalty: float | None = None,
-        seed: int | None = None,
-        stop: str | list[str] | None = None,
-        stream: bool | None = None,
-        stream_options: dict[str, Any] | None = None,
-        temperature: float | None = None,
-        top_p: float | None = None,
-        user: str | None = None,
-        guided_choice: list[str] | None = None,
-        prompt_logprobs: int | None = None,
-        suffix: str | None = None,
-    ) -> OpenAICompletion:
-        provider_model_id = await self._get_provider_model_id(model)
-
-        params = await prepare_openai_completion_params(
-            model=provider_model_id,
-            prompt=prompt,
-            best_of=best_of,
-            echo=echo,
-            frequency_penalty=frequency_penalty,
-            logit_bias=logit_bias,
-            logprobs=logprobs,
-            max_tokens=max_tokens,
-            n=n,
-            presence_penalty=presence_penalty,
-            seed=seed,
-            stop=stop,
-            stream=stream,
-            stream_options=stream_options,
-            temperature=temperature,
-            top_p=top_p,
-            user=user,
-        )
-
-        try:
-            return await self._client.completions.create(**params)
-        except APIConnectionError as e:
-            raise ConnectionError(f"Failed to connect to NVIDIA NIM at {self._config.url}: {e}") from e
-
-    async def openai_chat_completion(
-        self,
-        model: str,
-        messages: list[OpenAIMessageParam],
-        frequency_penalty: float | None = None,
-        function_call: str | dict[str, Any] | None = None,
-        functions: list[dict[str, Any]] | None = None,
-        logit_bias: dict[str, float] | None = None,
-        logprobs: bool | None = None,
-        max_completion_tokens: int | None = None,
-        max_tokens: int | None = None,
-        n: int | None = None,
-        parallel_tool_calls: bool | None = None,
-        presence_penalty: float | None = None,
-        response_format: OpenAIResponseFormatParam | None = None,
-        seed: int | None = None,
-        stop: str | list[str] | None = None,
-        stream: bool | None = None,
-        stream_options: dict[str, Any] | None = None,
-        temperature: float | None = None,
-        tool_choice: str | dict[str, Any] | None = None,
-        tools: list[dict[str, Any]] | None = None,
-        top_logprobs: int | None = None,
-        top_p: float | None = None,
-        user: str | None = None,
-    ) -> OpenAIChatCompletion | AsyncIterator[OpenAIChatCompletionChunk]:
-        provider_model_id = await self._get_provider_model_id(model)
-
-        params = await prepare_openai_completion_params(
-            model=provider_model_id,
-            messages=messages,
-            frequency_penalty=frequency_penalty,
-            function_call=function_call,
-            functions=functions,
-            logit_bias=logit_bias,
-            logprobs=logprobs,
-            max_completion_tokens=max_completion_tokens,
-            max_tokens=max_tokens,
-            n=n,
-            parallel_tool_calls=parallel_tool_calls,
-            presence_penalty=presence_penalty,
-            response_format=response_format,
-            seed=seed,
-            stop=stop,
-            stream=stream,
-            stream_options=stream_options,
-            temperature=temperature,
-            tool_choice=tool_choice,
-            tools=tools,
-            top_logprobs=top_logprobs,
-            top_p=top_p,
-            user=user,
-        )
-
-        try:
-            return await self._client.chat.completions.create(**params)
-        except APIConnectionError as e:
-            raise ConnectionError(f"Failed to connect to NVIDIA NIM at {self._config.url}: {e}") from e