Logprobs info to completion API (#806)

* Initial implementation

* Fix handling of return_step_logits in return

* Fixed OpenAI parameter expectations and logprob structure and datatypes

* pre-commit black formatting

* Remove unused parameter

* fix log probs

* fix colorize

* nits in server

* nits in server

* Fix top_logprobs structure (a dict) and include tokens in logprobs response

* nits

* fix types

---------

Co-authored-by: Awni Hannun <awni@apple.com>

llms/mlx_lm/SERVER.md

@@ -17,7 +17,7 @@ mlx_lm.server --model <path_to_model_or_hf_repo>
 For example:
 
 ```shell
-mlx_lm.server --model mistralai/Mistral-7B-Instruct-v0.1
+mlx_lm.server --model mlx-community/Mistral-7B-Instruct-v0.3-4bit
 ```
 
 This will start a text generation server on port `8080` of the `localhost`
@@ -73,4 +73,8 @@ curl localhost:8080/v1/chat/completions \
   applying repetition penalty. Defaults to `20`.
 
 - `logit_bias`: (Optional) A dictionary mapping token IDs to their bias
-  values. Defaults to `None`.
+  values. Defaults to `None`.
+
+- `logprobs`: (Optional) An integer specifying the number of top tokens and
+  corresponding log probabilities to return for each output in the generated
+  sequence. If set, this can be any value between 1 and 10, inclusive.