Speed up the video splitting when prepare the training data

### The original speed of video splitting in /easyanimate/video_caption/video_splitting.py is very slow, due to the 'ffmpeg' operation. **Improvement:** change the FFmpeg code in Line 17-32 in </easyanimate/video_caption/video_splitting.py> to: return [ 'ffmpeg', '-ss', str(start_time.time()), '-i', video_path, '-t', str(video_duration), '-c:v', 'copy', '-c:a', 'copy', output_path ] This will speed up the splitting process.