* add functions to copy images with validation and check the video frame number.
* update README to clarify image file handling and maintain metadata structure
* Update utils/dataset_merging/merge_lerobot_dataset.py
[feat] accelerate the episode file finding with the ` episode_file_mapping`.
* Refactor image and video handling in dataset merging tool with enhanced validation
* [fix] If `images` directory exists, then use`early_validation`
* fix: Import `encode_video_frames` for video encoding in early validation
* fix: Add image copying option and update README for PNG support
* fix: Remove unused cv2 import from dataset merging utility
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Co-authored-by: Qizhi Chen <tavish9.chen@gmail.com>
Co-authored-by: zhipeng tang <2444198418@qq.com>