Replace 'decord' with 'av' in VideoClassificationPipeline (#29747)
* replace the 'decord' with 'av' in VideoClassificationPipeline
* fix the check of backend in VideoClassificationPipeline
* adjust the order of imports
* format 'video_classification.py'
* format 'video_classification.py' with ruff
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Co-authored-by: wanqiancheng <13541261013@163.com>