mixtral evaluation PR
@ethanhe42   
too messy to fix the signature issue for the previous pr: https://github.com/NVIDIA/NeMo/pull/8988
So restart a new one. LOL..
 @Slyne could you move doc to `docs/source/multimodal/mllm/video_neva.rst` ? @ethanhe42 Done.
Please refer to the previous PR:
https://github.com/NVIDIA/NeMo/pull/8988