add Completions API support

[mattf]

        Create a new NVIDIA LLM for Completions APIs.

        This class provides access to a NVIDIA NIM for completions. By default, it
        connects to a hosted NIM, but can be configured to connect to a local NIM
        using the `base_url` parameter. An API key is required to connect to the
        hosted NIM.

        Args:
            model (str): The model to use for reranking.
            nvidia_api_key (str): The API key to use for connecting to the hosted NIM.
            api_key (str): Alternative to nvidia_api_key.
            base_url (str): The base URL of the NIM to connect to.

        API Key:
        - The recommended way to provide the API key is through the `NVIDIA_API_KEY`
            environment variable.

        Additional arguments that can be passed to the Completions API:
        - max_tokens (int): The maximum number of tokens to generate.
        - stop (str or List[str]): The stop sequence to use for generating completions.
        - temperature (float): The temperature to use for generating completions.
        - top_p (float): The top-p value to use for generating completions.
        - frequency_penalty (float): The frequency penalty to apply to the completion.
        - presence_penalty (float): The presence penalty to apply to the completion.
        - seed (int): The seed to use for generating completions.
        - best_of (int): The number of completions to generate and return the best of.
        - echo (bool): Whether to echo the prompt in the completion.
        - logit_bias (Dict[str, float]): The logit bias to apply to the completion.
        - logprobs (int): The number of logprobs to return.
        - n (int): The number of completions to generate.
        - suffix (str): The suffix to use for generating completions.
        - user (str): The user ID to use for generating completions.

        These additional arguments can also be passed with `bind()`, e.g.
        `NVIDIA().bind(max_tokens=512)`, or pass directly to `invoke()` or `stream()`,
        e.g. `NVIDIA().invoke("prompt", max_tokens=512)`.

[dglogo]

lgtm!

[dglogo]

Thanks for putting this together!