Repository: huggingface/lighteval
Issue: [BUG] The word `pretrained` is not accepted in `model_args` when using VLLM
Root cause: Validation Gap

## Describe the bug
In model_args, parameters not showing in [VLLMModelConfig](https://github.com/huggingface/lighteval/blob/main/src/lighteval/models/vllm/vllm_model.py#L75C23-L75C34) are not accepted. The `pretrained` parameter is not in the class. When I include it in the `model_args` when calling `lighteval vllm`, the error in the following figure is raised.

![Image](https://github.com/user-attachments/assets/8aef07c8-da33-4fdb-822c-63f3104f4cb7)

However, the parameter is essential when calling `lighteval vllm` since it helps lighteval build HF models in the function [`_create_auto_model`](https://github.com/huggingface/lighteval/blob/main/src/lighteval/models/vllm/vllm_model.py#L147C15-L147C15) in the class [`VLLMModel`](https://github.com/huggingface/lighteval/blob/main/src/lighteval/models/vllm/vllm_model.py#L98). Otherwise, VLLM does not know which model to load.

The main cause of the bug is that the class is inherited from [`ModelConfig`](https://github.com/huggingface/lighteval/blob/main/src/lighteval/models/utils.py#L38), which inherits from Pydantic's BaseModel with the setting `extra="forbid"`. The feature is introduced in the [pull request #609](https://github.com/huggingface/lighteval/pull/609).

## To Reproduce
Given the following bash shell script:

``` bash
TASK=gsm8k

NUM_GPUS=1
MODEL=Qwen/Qwen2.5-Math-7B-Instruct
MODEL_NAME=qwen2.5-math-7b-instruct
MODEL_ARGS="model_name=$MODEL_NAME,pretrained=$MODEL,dtype=float16,tensor_parallel_size=$NUM_GPUS,max_model_length=4096,gpu_memory_utilization=0.95"
OUTPUT_DIR=lighteval_results/$TASK/$MODEL_NAME
mkdir -p $OUTPUT_DIR
export VLLM_WORKER_MULTIPROC_METHOD=spawn
lighteval vllm $MODEL_ARGS "lighteval|gsm8k|0|0" \
    --use-chat-template \
    --system-prompt="Please reason step by step, and put your final answer within \\boxed{}." \
    --output-dir $OUTPUT_DIR \
    --save-details
```

By running the script, you will get an error as in the following figure:

![Image](https://github.com/user-attachments/assets/722b433d-5132-4cd0-a931-55ffb2fc67bf)

## Expected behavior
The expectation is that the `pretrained` parameter should be accepted by [VLLMModelConfig](https://github.com/huggingface/lighteval/blob/main/src/lighteval/models/vllm/vllm_model.py#L75C11-L75C11), and sent to the function [`_create_auto_model`](https://github.com/huggingface/lighteval/blob/main/src/lighteval/models/vllm/vllm_model.py#L147C15-L147C15) in the class [`VLLMModel`](https://github.com/huggingface/lighteval/blob/main/src/lighteval/models/vllm/vllm_model.py#L98).


## Version info

Operating system: Ubuntu 22.04
Lighteval version: 0.8.1.dev0
Pydantic version: 2.11.3
VLLM version: 0.8.3

Parameters other than `pretrained` required in the function [`_create_auto_model`](https://github.com/huggingface/lighteval/blob/main/src/lighteval/models/vllm/vllm_model.py#L147C15-L147C15) also need attention.

Also, the `generation_parameters` disappears in the [`VLLMModelConfig`](https://github.com/huggingface/lighteval/blob/main/src/lighteval/models/vllm/vllm_model.py#L75C23-L75C34) class as well.

The [document](https://github.com/huggingface/lighteval/blob/main/docs/source/use-vllm-as-backend.mdx) solves the issue. 

Thanks ! Just to be sure, we merged #609 recently and `generation_parameters` were move to a parent config class for models