Multi-gpu error with gradio from gpu_infer cast type: 'str' object cannot be interpreted as an integer

[pftq]

When running gradio with multi-gpu, it results in an error from gpu_infer being cast as a string because os.environ.get('LOCAL_WORLD_SIZE') returns a string. This PR fixes it by casting to int

After this, gradio works great with multi-gpu support:

gpu_count=2
python vace/gradios/vace_wan_demo.py \
--mp --ulysses_size ${gpu_count} --ring_size 1 \
--model_name "vace-14B" \
--ckpt_dir "models/Wan2.1-VACE-14B/"

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