Add distributed image captioning example

[ZAID646]

This PR adds a distributed image captioning example to accelerate.

Summary

• New script: examples/inference/distributed/distributed_image_captioning.py
• Demonstrates distributed inference using PartialState, split_between_processes, and gather_object
• Uses BLIP-2 (configurable) for image captioning with dataset inputs (Flickr30k by default)
• Serializes results as images + captions.json for downstream use

Related Issue

Closes #3078

What it shows

• How to split batches of images across multiple GPUs for caption generation
• How to gather string results across processes with gather_object
• How to serialize generated artifacts (images and caption metadata)

Usage

accelerate launch --num_processes {NUM_GPUS} examples/inference/distributed/distributed_image_captioning.py --batch_size 8