Add MEGA-Bench

[woodfrog]

Hi, lmms-eval team,

This PR incorporates our recent work, MEGA-Bench, a multimodal evaluation suite with over 500 real-world tasks and 45 metrics.

The evaluation process involves two steps: 1) run lmms-eval to produce the response/submission file; 2) run our evaluator with 45 diverse metrics to get the scores and multi-dimensional breakdown results.

Example response/submission file generation:

# Core set (440 tasks)
python3 -m accelerate.commands.launch \
    --num_processes=8 \
    -m lmms_eval \
    --model llava_onevision \
    --tasks megabench_core  \
    --batch_size 1 \
    --log_samples \
    --log_samples_suffix llava_ov_megabench_core \
    --output_path ./logs/ \
    --model_args=pretrained=lmms-lab/llava-onevision-qwen2-7b-ov,conv_template=qwen_1_5,model_name=llava_qwen

Detailed steps are recorded in this README file.

Please advise on the format of the README or other details, thanks!